Add grepdiff-status test to TESTS

This test verifies that grepdiff -s/--status correctly shows:
  + for file additions (old file is /dev/null)
  - for file removals (new file is /dev/null)
  ! for file modifications (both files exist)

The test also verifies that --empty-files-as-absent works correctly
with -s to treat files with empty old versions as additions rather
than modifications.

The test currently fails - it documents the correct expected behavior
that needs to be implemented.

Assisted-by: Claude Code

Author: twaugh

Makefile.am

@@ -225,6 +225,7 @@ TESTS = tests/newline1/run-test \
 	tests/grepdiff8/run-test \
 	tests/grepdiff9/run-test \
 	tests/grepdiff-original-line-numbers/run-test \
+	tests/grepdiff-status/run-test \
 	tests/number1/run-test \
 	tests/number2/run-test \
 	tests/number3/run-test \