[PENDING DEPENDENT] Test framework, part II

[apostasie]

This is on top of #3889 (on top of #3591) - should be merged in order.

This specific PR does cleanup and reorganize the testutil/test package.

Although this PR looks "large", it is not functionally changing anything (and is only touching tests and testing tools anyhow).

The key take-aways here are:

• move the new test module to its final destination (mod/tigron)
• split overcrowded module into more intuitive sub-packages:
  • expect (expect.Contains, expect.Match, expect.ExitCode)
  • require (require.Arm64, etc)
  • test (test.Case, etc)
• renamed some methods to more intuitive names (eg: test.Require becomes require.All)

A tigron is a tigon in french :) (the offspring of a tiger and a lion)

The next PR will focus on linting code with more stringent standards.