Add testing helpers that allow comparing diagnostics, refactor existing tests to use them.

[SarahFrench]

⚠️ This is a stacked PR, please look at #36455 ⚠️

This PR introduces testing helpers for comparing diagnostics in the tfdiags package.

Summary:

• There was a pre-existing helper in the terraform package -assertDiagnosticsMatch- that I've replaced in this PR.
• I've replaced use of cmp.Diff(want, got, DiagnosticComparer) with the helper that does that internally
• I've refactored code that purposefully converted Diagnostics to RPC-friendly versions into using the new helper, as the helper does that internally

Open questions:

• Which tests would want to check Sources?
• Which tests would want to assert the concrete type of a returned diagnostic?

In future these could also be consolidated and brought into the tfdiags package:

• assertDiagnosticSummary
  • func assertDiagnosticSummary(t *testing.T, diags hcl.Diagnostics, want string) bool {...} - implemented once
  • func assertDiagnosticSummary(t *testing.T, diags tfdiags.Diagnostics, want string) bool {...} - implemented once
• assertDiagnosticCount
  • func assertDiagnosticCount(t *testing.T, diags hcl.Diagnostics, want int) bool {...} - implemented in 2 places
  • func assertDiagnosticCount(t *testing.T, diags tfdiags.Diagnostics, want int) bool {...} - implemented once
• assertNoDiagnostics
  • func assertNoDiagnostics(t *testing.T, diags hcl.Diagnostics) bool {...} - implemented in 2 places
  • func assertNoDiagnostics(t *testing.T, diags tfdiags.Diagnostics) {...} - implemented in 2 places
  • func assertNoDiagnostics(t *testing.T, diags tfdiags.Diagnostics) bool {...} - implemented once
• assertExactDiagnostics
  • This is only implemented and used in package configs so this would be something to leave unchanged.
• assertNoErrors
  • func assertNoErrors(t *testing.T, diags tfdiags.Diagnostics) {...} - implemented twice

Target Release

N/A

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.