gh-35999: Run pytest for src tree if no filename is given
    
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This fixes the failure seen, for example,

https://github.com/sagemath/sage/actions/runs/5678761729/job/15389749593
?pr=35991

Note that the last failure message
```
local/var/lib/sage/venv-python3.8/lib/python3.8/site-
packages/matplotlib/tests/__init__.py:6: in <module>
    raise IOError(
E   OSError: The baseline image directory does not exist. This is most
likely because the test data is not installed. You may need to install
matplotlib from source to get the test data.
=========================== short test summary info
============================
ERROR  - OSError: The baseline image directory does not exist. This is
most likely because the test data is not installed. You may need to
install matplotlib from source to get the test data.
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection
!!!!!!!!!!!!!!!!!!!!
============================== 1 error in 12.78s
===============================
Error: Process completed with exit code 2.
```
This is not a fault of matplotlib! The fault lies on our pytest, which
blindly runs all tests found in the directory tree under the root dir.
(matplotlib test files are supposed to run in a development install,
where the baseline image directory exists, but our install (SPKG) of
matplotlib is not a development install, and hence there's no baseline
image directory.)

So we need to stop pytest from running blind. The failure in the
incremental test workflow results from an error in `sage-runtests`. The
relevant lines are
```python
        exit_code_pytest = 0
        import pytest
        pytest_options = []
        if args.verbose:
            pytest_options.append("-v")
        # #31924: Do not run pytest on individual Python files unless
        # they match the pytest file pattern.  However, pass names
        # of directories. We use 'not os.path.isfile(f)' for this so
that
        # we do not silently hide typos.
        filenames = [f for f in args.filenames
                     if f.endswith("_test.py") or not os.path.isfile(f)]
        if filenames or not args.filenames:   #
<------------------------------------ faulty line!
            print(f"Running pytest on {filenames} with options
{pytest_options}")
            exit_code_pytest = pytest.main(filenames + pytest_options)
            if exit_code_pytest == 5:
                # Exit code 5 means there were no test files, pass in
this case
                exit_code_pytest = 0
```
Look at the faulty line. The condition test allows pytest run without
explicit filenames. This results in running pytest for all test files
found under the (sage) root dir. Hence pytest attempts to run tests in
matplotlib in `local/var/lib/sage/venv-python3.11/lib/python3.11/site-
packages/matplotlib/tests`.

The branch of this PR fixes the problem.

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### ⌛ Dependencies

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URL: #35999
Reported by: Kwankyu Lee
Reviewer(s): Kwankyu Lee, Matthias Köppe, Tobias Diez

Author: Release Manager

src/bin/sage-runtests

@@ -175,13 +175,18 @@ if __name__ == "__main__":
         pytest_options = []
         if args.verbose:
             pytest_options.append("-v")
-        # #31924: Do not run pytest on individual Python files unless
-        # they match the pytest file pattern.  However, pass names
-        # of directories. We use 'not os.path.isfile(f)' for this so that
-        # we do not silently hide typos.
-        filenames = [f for f in args.filenames
-                     if f.endswith("_test.py") or not os.path.isfile(f)]
-        if filenames or not args.filenames:
+
+        # #35999: no filename in arguments defaults to "src"
+        if not args.filenames:
+            filenames = [SAGE_SRC]
+        else:
+            # #31924: Do not run pytest on individual Python files unless
+            # they match the pytest file pattern.  However, pass names
+            # of directories. We use 'not os.path.isfile(f)' for this so that
+            # we do not silently hide typos.
+            filenames = [f for f in args.filenames
+                         if f.endswith("_test.py") or not os.path.isfile(f)]
+        if filenames:
             print(f"Running pytest on {filenames} with options {pytest_options}")
             exit_code_pytest = pytest.main(filenames + pytest_options)
             if exit_code_pytest == 5: