tests: kernel: context: Fix test_busy_wait timeout

The commit b7e36366 added an additional
busy wait call in the `busy_wait_thread` function -- effectively making
the minimum time required for the thread to exit twice that of the
original implementation.

This commit updates the busy wait thread completion timeout to reflect
that change.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>