tests: schedule_api: fix variable shadowing

The array of k_thread "t" was declared non-static in 2 different
C files. Make them static.

Semaphores only used in local C file now declared static.

Use of variable 't' in thread_tslice() no longer shadows global
definition.

Fixes build errors with XCC compiler.

Increase RAM requirement to 20K.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>