gdb/testsuite: Remove duplicates from gdb.threads/staticthreads.ex

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.threads/staticthreads.exp ...
    DUPLICATE: gdb.threads/staticthreads.exp: couldn't compile staticthreads.c: unrecognized error

Fix by using foreach_with_prefix instead of foreach when preparing the
test case.

Testeed on x86_64-linux both in a setup where the test fails to prepare
and in a setup where the test fails to setup.