lib / string_helpers: refactoring the test suite

lib / string_helpers: refactoring the test suite

This patch prepares test suite for a following update.  It introduces
test_string_check_buf() helper which checks the result and dumps an error.

Signed-off-by: Andy Shevchenko <[email protected]>
Cc: "John W . Linville" <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>