[flang][test] Share all driver test dirs between `f18` and `flang-new`

Originally, when we added the new driver, we created dedicated test
directories for `flang-new`. This way we separated the tests for the
`throwaway` and the new driver.

As we are increasing test coverage and starting to share tests between
the two drivers, it makes sense to share all directories and instead
rely on:
```
! REQUIRES: new-flang-driver
```
to mark tests as exclusively for the new driver.

Differential Revision: https://reviews.llvm.org/D97207