Single-compile testing framework

[JustusAdam]

What Changed?

Adds the InlineTestBuilder which can produce a PreFrg compilation result from a string of source code. Integrates with the existing unit test infrastructure.

Why Does It Need To?

This allows to define test cases without needing external files (as used by the current define_test! template) or temporary directories (as used by the Test integration test framework) and more importantly the source code for the test inputs is compiled as-needed, which means e.g. tracing output is only reported for the selected test case, and that when test cases are filtered the tests run faster.

Checklist

• Above description has been filled out so that upon quash merge we have a
  good record of what changed.
• New functions, methods, types are documented. Old documentation is updated
  if necessary
• Documentation in Notion has been updated
• Tests for new behaviors are provided
  • New test suites (if any) ave been added to the CI tests (in
    .github/workflows/rust.yml) either as compiler test or integration test.
    Or justification for their omission from CI has been provided in this PR
    description.