tests: cram: test_base64: fix failing tests

tests: cram: test_base64: fix failing tests

Seems like latest version of llvm compiler/sanitizer has changed
behaviour during crash so `Aborted (core dumped)` is now printed to
stdout.

Fixes following issue:

 --- /builds/openwrt/project/libubox/tests/cram/test_base64.t
 +++ /builds/openwrt/project/libubox/tests/cram/test_base64.t.err
 @@ -49,9 +49,7 @@
    b64_encode: Assertion `dest && targsize > 0' failed.

    $ test-b64_decode-san 2> output.log; check
 -  Aborted (core dumped)
    b64_decode: Assertion `dest && targsize > 0' failed.

    $ test-b64_encode-san 2> output.log; check
 -  Aborted (core dumped)
    b64_encode: Assertion `dest && targsize > 0' failed.

References: https://gitlab.com/openwrt/project/libubox/-/jobs/1069840314
Signed-off-by: Petr Štetiar <ynezz@true.cz>