GDB: testsuite: MI: Don't return value from top-level (manual)

This patch manually changes "return -1" statements that weren't caught
by the sed command.  It also changes a couple of "return 0" statements.

These return statements fall into one of these criteria:

- misaligned line, which is then fixed by this patch;
- return at top level but inside block such as with_test_prefix,
  foreach_with_prefix, gdb_expect.

Approved-By: Tom Tromey <tom@tromey.com>