No filenamrs is equivalent to src/sage

Author: kwankyu

src/bin/sage-runtests

@@ -171,12 +171,17 @@ 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)]
+
+        # #35999: no filenames in arguments is equivalent to "src/sage"
+        if not args.filenames:
+            filenames = [os.path.join(os.environ.get('SAGE_SRC'), "sage")]
+        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)