gdb/testsuite: don't overwrite test executable in py-frame.exp

In gdb.python/py-frame.exp when we re-compile the test executable
without debug information, give it a new, unique, name to make
re-running the tests outside the testsuite easier.

While I was in the area I removed a `gdb_exit` call that was not
necessary, `prepare_for_testing` calls `clean_restart` which already
does a `gdb_exit`.

There should be no change in what is being tested after this commit.