test: refactor test-crypto-pbkdf2

This simplifies the test case by introducing a helper function that simply verifies that the sync and async variants produce the same result.

Also, testPBKDF2 did not use mustCall()/mustSucceed(), so it would not fail if the callback was never called.

This effectively also changes the loose mustCall() check at the end of the file to the stricter mustSucceed() function.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines