Change the existing strstr() test a little so that the substring not found is "bits", i.e. one that is actually found when doing case insensitive search. Then copy all of lib_strstr(), adapt the expectation for the strcasestr(s1, s3) result, and add another "not found" case. Reviewed-by: Simon Glass <sjg@chromium.org> Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>