Tests: Replace uses of deprecated AbsolutePath initializers with throwing variant

[tshortli]

AbsolutePath(_:) has been replaced by a throwing alternative AbsolutePath(validating:). There were dozens of warnings about this emitted when building the driver making it difficult to notice newly introduced warnings.

[tshortli]

@swift-ci please test

[artemcm]

Thanks! I have had this for a while:
#1231
And @compnerd asks we run the tests on Windows before merging. I've been slowly setting up my environment to do so...

[tshortli]

Ah, we can take your PR if you prefer it @artemcm. I'm a bit confused about the request to run on Windows, though. This change should not be able to change anything about the results of the tests on Windows given that the deprecated initializer just calls the throwing intializer and we're removing that indirection. There's zero functional change, just removal of a passthrough layer of indirection:

// MARK: - `AbsolutePath` backwards compatibility, delete after deprecation period.

extension AbsolutePath {
    @available(*, deprecated, message: "use throwing variant instead")
    public init(_ absStr: String) {
        try! self.init(validating: absStr)
    }

    @available(*, deprecated, message: "use throwing variant instead")
    public init(_ str: String, relativeTo basePath: AbsolutePath) {
        try! self.init(validating: str, relativeTo: basePath)
    }
}

[compnerd]

I agree with @tshortli that this should be okay (but being able to run the Windows tests would be nice). Hopefully we can get there soon!

[tshortli]

I'll defer to @artemcm's PR as he's got some different approaches in a few places, I should have checked if there was already something in progress for this.