tests/kernel: Bump kobj thread bitmask size for a few tests

A scheduler fix for free memory usage on aborted threads is now using
a per-CPU dummy thread instead of a single stack-based one at startup.
These static thread objects need spots in the kobj bitmasks, and a few
tests are sitting right at the default limit (16 threads).

Signed-off-by: Andy Ross <andyross@google.com>