Add a cecil test to compare [Availability] attributes (legacy) with [SupportedOSPlatform] (net6)

[spouliot]

Original discussion
https://discord.com/channels/732297728826277939/732297808148824115/879993217762857010

Background

• net5 introduced new custom attributes for API availability
• Our binding generator is automagically converting the legacy, [Availability]-based, attributes into [SupportedOSPlatform]
• Not everything we ship is generated, we have manual bindings (e.g. p/invokes and helpers) that needs to have their attributes updated.
• We have several introspection tests to validate [Availability]-based attributes.

Problem

• Manual bindings updates are done manually, with some helpers (macros). This has potential for adding some mistakes, even if each PR is reviewed by one (or often many) peer(s).
• Our tests, even if executed on older devices/simulators, cannot cover 100% of the cases, e.g. a cases like [iOS (12,3)] on an API that was added in [iOS (12,1)] would not be spotted by runtime test checks.

Potential Solutions

The real question is when to do something...

• now
• later... or
• never

Do nothing

Why ?

• Most API annotations are generated.
• Most existing API annotations are correct
  • where correct means it match Apple documentation or, since it's often incorrect, is based on runtime tests
  • incorrect annotations that pass our existing tests will remain (incorrect)
• Most of the PR updates are/will be correct
  • running intro on older devices/simulators will spot many (but not all) type of errors
• Updating the attributes is not a breaking change (source or binary) if/when mistakes are found.

Add New Unit Tests

Each set of attributes, legacy and dotnet, exists in separate assemblies. Comparing them requires loading both so that would make it (easier to be a) cecil-based test.

Still this will not be perfect. This will only ensure that the legacy attributes are a match with the new net6 attributes (IOW GIGO).

Lifetime: This will be useful as long as legacy is supported.

[chamons]

I believe this is effectively covered by the tests in

• [tests] Add disabled NET6 availability attribute test #14106
• [NET Attribute Conversion] Add missing attributes in code-behind by re-running conversion #14660
  and now
• [NET Attribute Conversion][generator] Generate NET style attributes #14779

I don't actually compare legacy to NET6, but apply validation logic that even caught a significant number of incorrect legacy binding attributes.

As I continue testing on #10170 and the cleanup issues in #14802 I will add additional tests as I'm able.