tests: posix: timers: provide at least 1 dynamic thread

timer_create() will error if there are no dynamic threads to
run the SIGEV_THREAD handler, causing the test to fail (where
it was previously expected to pass).

Ensure that there is at least 1 dynamic thread available to
the POSIX API.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>