test(conftest): make cwd in @pytest.mark.bashcomp relative from fixtures

@pytest.mark.bashcomp(cwd=path), which has been recently added in
commit 2ed0e85, had interpreted "cwd" as the relative path to the
pytest's working directory.  Now this commit makes "cwd" the relative
path to the directory "test/fixtures".

Note that @pytest.mark.bashcomp(cwd=path) has never been used so far
since this is added just for completeness when
@pytest.mark.bashcomp(temp_cwd=True) was supported.  This will be used
for the subsequent commits for the completion of "make".

test/t/conftest.py

@@ -241,7 +241,9 @@ def bash(request) -> pexpect.spawn:
         cwd = None
         if marker:
             if "cwd" in marker.kwargs and marker.kwargs.get("cwd") is not None:
-                cwd = marker.kwargs.get("cwd")
+                cwd = os.path.join(
+                    testdir, "fixtures", marker.kwargs.get("cwd")
+                )
             elif "temp_cwd" in marker.kwargs and marker.kwargs.get("temp_cwd"):
                 tmpdir = tempfile.TemporaryDirectory(
                     prefix="bash-completion-test_"