subsys/testsuite: Shorten the assertion messages

At present these can be very long since they include the full path of
the filename with the error.

    Assertion failed at /home/sglass/cosarm/zephry/zephyrproject/
	zephyr/tests/drivers/flash_simulator/src/main.c:102:
	test_write_read: (0 not equal to rc)

Reduce the length by omitting the current directory (where the tests
are being run) from the output:

    Assertion failed at tests/drivers/flash_simulator/src/main.c:102:
	test_write_read: (0 not equal to rc)

This improves usability for people running tests locally.

Signed-off-by: Simon Glass <sjg@chromium.org>