Fix TEST_UNSKIP in pass_rate (#3030)

Do not require `TEST_UNSKIP` in `pass_rate.py`, assume it is false by
default.

Fixes #3029.

Author: pbchekin

scripts/pass_rate.py

@@ -103,7 +103,7 @@ def parse_report(report_path: pathlib.Path, skiplist_dir: pathlib.Path) -> Repor
             pass
         stats.fixme += len(testsuite_fixme_tests)
 
-    test_unskip = os.getenv('TEST_UNSKIP')
+    test_unskip = os.getenv('TEST_UNSKIP', 'false')
     if test_unskip not in ('true', 'false'):
         raise ValueError('Error: please set TEST_UNSKIP true or false')
     if test_unskip == 'false':