[flang] Clean up temp dirs

A temp directory is created in `common.sh` and it is cleaned up by
`trap ... EXIT`. If the test script has its own trap, as this one does,
it seems to replace the first one. So the cleanup from `common.sh` was
not being executed when the `%t` feature was used and empty temp
directories were being left in the directory where the tests ran.

Since we already have a temp directory that is cleaned up, just use
that for `%t` and don't bother with another `mktemp`.

Original-commit: flang-compiler/f18@f61d62ddec011c1a6462ece12af29c5f0b16e60a
Reviewed-on: https://github.com/flang-compiler/f18/pull/754