test: Use positive conditional in test_matches()

It is easier to read the positive conditional.

While at it, convert hard coded length of "_test_" to strlen("_test_")
which will be converted to a constant bu optimizing compiler.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>