We want to check that the async onDecrypt function returns a promise
that rejects.

We were trying to use chai-as-promised's rejectedWith matcher for this,
but within an `async` function.

It looks like mocha isn't clever enough to wait for the promise to
reject and run the assertion unless we return the promise from the
function. It prints a warning about the uncaught promise rejection
instead.

Once we fix the test to return the promise it fails, because the promise
is not actually being rejected, so I'm excluding it while I work on a
fix.

This test was excluded because the precondition wasn't hit when calling
`keyring.onDecrypt` (I think due to another precondition somewhere
else).

It might be a bit sneaky to call the "private" method (`_unwrapKey`) in
the tests, but it hits the precondition we're trying to test more
directly.

This is particularly useful because CloudFront's Field Level Encryption
uses RSA_OAEP_SHA256_MGF1, which this library doesn't support yet.

Support for oaepHash was added in node 12.9 (nodejs/node#28335), so this
won't work for older node versions. It's still a backwards compatible
change because by default `oaepHash` will be undefined, as before.

I've updated the tests to cover use of the new parameter, but they're
not very strict because they both encrypt and decrypt using the same
parameter.

This means if node silently ignores the oaepHash parameter (as it will
in versions < 12.9) the tests will still pass, which isn't great.

On the other hand, I think this project may still be being tested on an
older version of node, so perhaps the fact the tests won't break is an
unexpected blessing.

I've also tested this manually against AWS CloudFront's Field Level
Encryption and it seems to work.

Resolves aws#198

Judging from the types in the test, this is supported by other
libraries, and it looks like it also works in node.

I haven't been able to run these tests myself to confirm they work
because there's a fair bit of setup I don't know how to do.

I'd expect the tests to fail on versions of node < 12.9, which may be a
problem for CI.

It is important to be perscriptive in what options will work.
Node.js versions that do not support `oaepHash` will silently encrypt data.
This means that the encrypted data key would not have the security properties requested.
So, `oaep_hash_supported.ts` will attempt to encrypt
and report the success.
This will happen only once, on initializaion.

Both the tests, and the integration tests have been updated honor `oaepHashSupported` values