[flang][test] Don't require .exe suffix.

The suffix is only added then the path the (lld-)link.exe is fully
resolved. Otherwise it may stay "link" or "lld-link".

On non-Windows platforms, lld-link also exists as a symbolic link to
lld, such that the full the path to lld-link might also be resolved
without .exe suffix in this case.

Note that CLANG_DEFAULT_LINKER could be set to other linkers such as
mold, in which case the test may still fail. This also applies to the
non-Windows tests that require "ld" in the linker name.

Fixes issue #56955