gh-39102: Show continuation lines when doctest crashes
    
I think this makes more sense.

Normally the format is like

```
sage: code(code)  ## line 1234 ##
'result'
```

or

```
sage: code(code)
code(code)  ## line 1234 ##
'result'
```

with the patch, the display becomes

```
sage: code(code)
....: code(code)  ## line 1234 ##
'result'
```


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #39102
Reported by: user202729
Reviewer(s): Julian Rüth

Author: Release Manager

.github/workflows/build.yml

@@ -107,6 +107,8 @@ jobs:
         uses: tj-actions/changed-files@v45
         with:
           # File extensions for doctests per sage.doctest.control.skipfile
+          # Also src/sage/doctests/tests/ are excluded because of nodoctest file
+          # which would make sage.doctest.control.skipdir return True
           files_yaml: |
             configures:
               - 'build/pkgs/*/spkg-configure.m4'
@@ -118,6 +120,7 @@ jobs:
             doctests:
               - 'src/**/*.{py,pyx,pxd,pxi,sage,spyx,rst,tex}'
               - '!src/{setup,conftest*}.py'
+              - '!src/sage/doctest/tests/*'
 
       - name: Determine targets to build
         id: build-targets
@@ -246,7 +249,7 @@ jobs:
           ./sage -python -m pytest -c tox.ini -qq --doctest --collect-only || true
         shell: sh .ci/docker-exec-script.sh BUILD /sage {0}
 
-      - name: Test changed files (sage -t --new)
+      - name: Test changed files
         if: (success() || failure()) && steps.container.outcome == 'success' && steps.changed-files.outputs.doctests_all_changed_files
         run: |
           export MAKE="make -j2 --output-sync=recurse" SAGE_NUM_THREADS=4

src/sage/doctest/forker.py

@@ -664,7 +664,8 @@ def _run(self, test, compileflags, out):
             # We print the example we're running for easier debugging
             # if this file times out or crashes.
             with OriginalSource(example):
-                print("sage: " + example.source[:-1] + " ## line %s ##" % (test.lineno + example.lineno + 1))
+                assert example.source.endswith("\n"), example
+                print("sage: " + example.source[:-1].replace("\n", "\n....: ") + " ## line %s ##" % (test.lineno + example.lineno + 1))
             # Update the position so that result comparison works
             self._fakeout.getvalue()
             if not quiet:

src/sage/doctest/test.py

@@ -295,15 +295,21 @@
     ...
     16
 
-A different kind of crash::
+A different kind of crash (also test printing of line continuation ``...:``,
+represented by ``<DOTSCOLON>`` below)::
 
-    sage: subprocess.call(["sage", "-t", "--warn-long", "0",    # long time
-    ....:      "--random-seed=0", "--optional=sage", "fail_and_die.rst"], **kwds)
+    sage: # long time
+    sage: proc = subprocess.run(["sage", "-t", "--warn-long", "0",
+    ....:      "--random-seed=0", "--optional=sage", "fail_and_die.rst"], **kwds,
+    ....:      stdout=subprocess.PIPE, text=True)
+    sage: # the replacements are needed to avoid the strings being interpreted
+    ....: # specially by the doctesting framework
+    sage: print(proc.stdout.replace('sage:', 'sage<COLON>').replace('....:', '<DOTSCOLON>'))
     Running doctests...
     Doctesting 1 file.
     sage -t --warn-long 0.0 --random-seed=0 fail_and_die.rst
     **********************************************************************
-    File "fail_and_die.rst", line 5, in sage.doctest.tests.fail_and_die
+    File "fail_and_die.rst", line 8, in sage.doctest.tests.fail_and_die
     Failed example:
         this_gives_a_NameError
     Exception raised:
@@ -313,11 +319,18 @@
         Killed due to kill signal
     **********************************************************************
     Tests run before process (pid=...) failed:
-    ...
+    sage<COLON> import time, signal ## line 4 ##
+    sage<COLON> print(1,
+    <DOTSCOLON>       2) ## line 5 ##
+    1 2
+    sage<COLON> this_gives_a_NameError ## line 8 ##
+    sage<COLON> os.kill(os.getpid(), signal.SIGKILL) ## line 9 ##
+    **********************************************************************
     ----------------------------------------------------------------------
     sage -t --warn-long 0.0 --random-seed=0 fail_and_die.rst  # Killed due to kill signal
     ----------------------------------------------------------------------
     ...
+    sage: proc.returncode
     16
 
 Test that ``sig_on_count`` is checked correctly::

src/sage/doctest/tests/fail_and_die.rst

@@ -1,6 +1,9 @@
 The :exc:`NameError` raised on the second line should be displayed, even
-if we crash immediately afterwards::
+if we crash immediately afterwards (also test printing of line continuation)::
 
     sage: import time, signal
+    sage: print(1,
+    ....:       2)
+    1 2
     sage: this_gives_a_NameError
     sage: os.kill(os.getpid(), signal.SIGKILL)