Additional tests for sha1 and sha256 compression.

* testsuite/sha1-test.c (test_sha1_compress): New function.
(test_main): Add tests for compressing 0, 1 or 2 blocks.
* testsuite/sha256-test.c (test_sha256_compress): New function.
(test_main): Add tests for compressing 0, 1 or 2 blocks.