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

This patch manually changes "return -1" 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 block such as for loops,
  with_test_prefix, foreach_with_prefix, gdb_test_multiple.

I also fixed a couple cases of "return 0" from top-level that also
weren't caught by sed, and even a couple cases of "return 1" from
top-level.

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