gh-109402: Fix libregrtest when module matches folder name

[sobolevn]

It is fine now :)

0:00:00 load avg: 2.42 Run 5 tests sequentially
...
0:00:00 load avg: 2.42 [5/5] test_future_stmt.test_future_stmt

== Tests result: SUCCESS ==

All 5 tests OK.

Total duration: 177 ms
Total tests: run=32
Total test files: run=5/5
Result: SUCCESS

I am not sure if I should test it somehow in CI.

• Issue: Bug in regrtest when name in SPLITTESTDIRS also matches a module name in tests #109402