[flang][driver] Rename driver tests (nfc)

As per [1]:
```
File names should use dashes, not underscores.
```

This patch updates the names of Flang driver tests accordingly.

[1] https://github.com/llvm/llvm-project/blob/main/flang/docs/C%2B%2Bstyle.md

diff --git a/flang/test/Driver/no_files.f90 b/flang/test/Driver/no-files.f90
similarity index 100%
rename from flang/test/Driver/no_files.f90
rename to flang/test/Driver/no-files.f90

diff --git a/flang/test/Driver/version_test.f90 b/flang/test/Driver/version-test.f90
similarity index 100%
rename from flang/test/Driver/version_test.f90
rename to flang/test/Driver/version-test.f90

diff --git a/flang/test/Flang-Driver/macro_def_undef.f90 b/flang/test/Flang-Driver/macro-def-undef.f90
similarity index 100%
rename from flang/test/Flang-Driver/macro_def_undef.f90
rename to flang/test/Flang-Driver/macro-def-undef.f90

diff --git a/flang/test/Flang-Driver/macro_multiline.f90 b/flang/test/Flang-Driver/macro-multiline.f90
similarity index 100%
rename from flang/test/Flang-Driver/macro_multiline.f90
rename to flang/test/Flang-Driver/macro-multiline.f90