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

This patch manually changes "return -1" and "return 0" statements that
weren't caught by the sed command.

These return statements fall into one of these criteria:

- misaligned line, which is then fixed by this patch;
- return at top level but inside a block such as save_vars,
  with_test_prefix, foreach, gdb_test_multiple.

Some weren't changed because they didn't meet the criteria above, or
weren't trivial to check.

There was also one return statement with an unnecessary semi-colon at
the end, which was removed.

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