testing: expectDiagnostics() should provide an option to ignore ordering

[garethj-msft]

Clear and concise description of the problem

Overview

Code churn can easily cause diagnostics to appear in changing orders over time, and this should, in general, not be considered a breaking change.

However, library tests that use this helper function will fail when diagnostic order changes.
It's onerous for every library test author to have to add a wrapper to pre-sort the collections; rather the core library should have a mode that enables this with a simple binary switch, as an addition to the options object.

The existing strict option could be used, but that would technically be a behavioral breaking change to the test condition.

Current

compiler/expect.ts/expectDiagnostics(
  diagnostics: readonly Diagnostic[],
  match: DiagnosticMatch | DiagnosticMatch[],
  options = {
    strict: true,
  });

Expected

compiler/expect.ts/expectDiagnostics(
  diagnostics: readonly Diagnostic[],
  match: DiagnosticMatch | DiagnosticMatch[],
  options = {
    strict: true,
    fixedOrder: true
  });

Checklist

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• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[timotheeguerin]

That would be nice but I think in general it would be better practice to make sure your test only emit a single diagnostic.
Not doing order makes it also potentially harder to see what isn't matching(would just be able to say didn't find this exect diagnostic in this list).

[markcowl]

Have expectDiagnostics sort the diagnostics