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 <[email protected]>

Author: cfriedt

tests/posix/timers/prj.conf

@@ -3,3 +3,7 @@ CONFIG_ZTEST=y
 
 CONFIG_POSIX_AEP_CHOICE_BASE=y
 CONFIG_POSIX_TIMERS=y
+
+# Needed for timer_create() when using SIGEV_THREAD
+CONFIG_DYNAMIC_THREAD=y
+CONFIG_DYNAMIC_THREAD_POOL_SIZE=1