fix failing RSA test 'testHandlingNonStandardKeys'

[Sajjon]

Fix failing test testHandlingNonStandardKeys

Possible regression from https://github.com/apple/swift-crypto/pull/151 ? The tests failed for me when I set let development = true in Package.swift and run target "My Mac" running 13.5 Beta (22G5059d) on a Macbook M1.

Checklist

• [x] I've run tests to see all new and existing tests pass
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

If you've made changes to gyb files

• [ ] I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

Tests should not fail.

Modifications:

Changed from XCTAssertEqual which failed to XCTAssertThrowsError to capture the fact that the test tests PEM and DER representations which were not a multiple of 8 bits in length.

Result:

All tests pass.

[Lukasa]

This is not a regression but a behavioural change, see #185. A fix has been discussed there.