kunit_tool: add support to print only sample test Add a new flag - --print_test_only that only outputs the sample -test.c file to stdout. This is to facilitate the creation of the corresponding vim plugin. Change-Id: Id7b212337472aff27037f00f824f757663f89159 Google-Bug-Id: 117337457 Signed-off-by: Pallav Agarwal <pallavag@google.com>