[flang][nfc] Port 2 tests to use the new driver when enabled

This is similar to https://reviews.llvm.org/D100309, i.e. `%f18` is
replaced with `%flang_new`.

resolve105.f90 wasn't in tree when D100309 was worked on, so it's
updated here instead.

label14.f90 requires `-fsyntax-only`. I didn't notice that when
submitting D100309, hence updating it now instead. `-fsyntax-only` is
required to prevent `%f18` from calling an external compiler (which then
fails and returns a non-zero exit code).

Differential Revision: https://reviews.llvm.org/D100655