Add a comment to the platform #ifdef forrest.
This test is timing out again. Disabled.
Add a unit test for find_bytes.

When run under valgrind (./configure --enable-valgrind && make check)
this test shows the invalid read problem reported in
https://github.com/mozilla/webvtt/issues/21

The find_bytes() function is static, so the only way test it is
with #ifdef'd code in the same source file. Unfortunately parser.o
won't link on its own. I uses symbols from most of the other source
files as well.

Probably find_bytes should move to a separate source file or
string.c, and be WEBVTT_INTERNAL so we can call it from the
gtest framework in test/unit.
Shayan's fixed find_bytes() from issue #21.

Moving the conditional to the start of the loop avoids an
invalid read if len < slen.
More verbose output from the find_bytes() unit test.

Doesn't seem to work? Should only show/not show a problem
under valgrind for the current implementation.