siso: add tests for filegroup glob matching and starlark parsing

Previously, filegroup tests only covered basic filename-only patterns,
leaving key functionality untested:
- The code path for patterns containing '/' (which matches against the
  full relative path using path.Match instead of just path.Base) was
  never tested for both includes and excludes.
- Handling of absolute root directories (for CrOS chroot), invalid
  paths outside the workspace, and non-existent directories lacked
  test coverage.
- Starlark configuration parsing and Config.UpdateFilegroups in
  star_filegroups.go had 0% test coverage.

This CL adds comprehensive test cases to address these gaps:
- Test pattern matching with slash in includes (e.g. "debug/*.h") and
  excludes (e.g. "debug/*").
- Test absolute directory prefixing, invalid workspace paths, and zero-
  match cases in TestFilegroupGlob.
- Add TestParseFilegroups to verify parsing of implicit/explicit 'dir',
  'includes', 'excludes', and invalid configuration errors.
- Add TestUpdateFilegroups to verify concurrent updates and handling of
  missing directories (fs.ErrNotExist).

Change-Id: I54d8237903c60645ed196a4ec9b1123deb478892
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8257189
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>