Fix p_test_fail()'s va_list abuse.

p_test_fail() was passing a va_list to two separate functions with the
expectation that no reset would occur.  Refactor p_test_fail()'s callers
to instead format two strings and pass them to p_test_fail().

Add a missing parameter to an assert_u64_eq() call, which the compiler
warned about after the assertion macro refactoring.