Bignum tests: use TEST_EQUAL
Bignum tests: use TEST_EQUAL

TEST_EQUAL(), has the benefit of outputting the values that don't match,
which can make debugging simpler.

Scope:
- Recently added new test functions
- Checks making sure the test case/data is consistent is out of scope
- Only checks where printing the values is likely to be helpful

Signed-off-by: Janos Follath <b175c17abe0474719672be1367bb75ee28bd5c71@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls