gdb/testsuite: resolve some duplicate testnames in gdb.mi

Set of fixes to resolve some duplicate test names in the gdb.mi/
directory.  There should be no real test changes after this set of
fixes, they are all either:

  - Adding with_test_prefix type constructs to make test names unique,
    or

  - Changing the test name to be more descriptive, or better reflect
    the test being run.