Avoid ObjC runtime warnings about duplicate class definitions for Serializer type

[stmontgomery]

This is a workaround meant to avoid Objective-C runtime warnings about duplicate classes stemming from our internal Serializer type which was added in #1390.

Motivation:

In some usage scenarios there can be two or more copies of the testing library loaded into a runner process. When this happens on platforms such as Darwin which use the Objective-C runtime, this can cause duplicate class definition warnings logged to the console because class types and (non-generic) actor types are implemented within the Objective-C runtime as classes. This can look something like the following:

objc[44611]: Class _TtC7Testing10Serializer is implemented in both ../path/to/libTesting.dylib (0x1011ac050) and path/to/swift-testing/.build/arm64-apple-macosx/debug/swift-testingPackageTests.xctest/Contents/MacOS/swift-testingPackageTests (0x10cc3ab88). This may cause spurious casting failures and mysterious crashes. One of the duplicates must be removed or renamed.

Modifications:

I considered several potential fixes, but decided the simplest and most likely to work in all usage scenarios is to make the Serializer type generic by adding an unused generic type parameter T.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.