Fix exp function to actually work

[djrodgerspryor]

It was calling into decNumberExp with the wrong signature, so it didn't work at all.

I also fixed the decNumber tests for exp so that they'll actually run if point at the exp.decTest file, and I added a super basic test for ln and exp.

It seems a bit too easy to not run some of the decNumber test by accident right now; maybe it would be good to expect every test file to have some non-igored tests, and then just delete the test files for actually unimplemented functionality?

[alkis]

Why do we need this? This test should not change at all.

[djrodgerspryor]

Because of these lines in the run-test script:

    let d128_env = Environment {
        precision: Some(34),
        rounding: Some(Rounding::HalfEven),
        max_exponent: Some(6144),
        min_exponent: Some(-6143),
        extended: true,
        clamp: true,
    };

    if *env != d128_env {
        return TestResult::Ignored(test);
    }

Without that change, the tests were silently skipped (and none of the other matched either).

Maybe I should have gone in the other direction and defined a different Environment just for these tests? I assume that the check is there to make sure that it only runs applicable tests, but since the exp tests pass with the different clamping mode, I assume that all of those test cases are still valid with the different clamping mode.

[alkis]

Is it possible to change run_test() to run ops with the specified environment which will make this change unnecessary and increase test coverage?