gdb/testsuite: remove duplicates from gdb.base/stap-probe.exp

Remove the duplicate test names from gdb.base/stap-probe.exp, this is
done by actually passing a unique test name in a couple of
places (rather than using the command as the test name), and in
another couple of places, a test has a duplicate name due to a cut &
paste error, which I've fixed.

There's no change in what is actually being tested after this commit.