Convert all unit tests to Swift Testing

[mbernson]

This PR converts all of the SwiftGit2 unit tests from Quick & Nimble to the new Swift Testing framework.

I've taken care to not change the logic, but only translate all the existing tests to equivalent Swift Testing tests. (Using Suites, #require, #expect, etcetera).

The large number of lines changes is only because there is one less level of indentation in the test specs now:

Before

class OIDSpec: QuickSpec {
    override class func spec() {
        // Tests
    }
}

After

@Suite("OID") class OIDSpec {
    // Tests
}