test: string: add test of new strcasestr() function

test: string: add test of new strcasestr() function

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>