Allow execution of non-feature tests

[crandles]

Support the execution of tests without requiring use of the features package.

To the Environment interface, I propose the addition of two new functions:

type Environment interface {
	RunTest(ctx context.Context, t *testing.T, cfg *envconf.Config) bool
	RunBenchmark(ctx context.Context, b *testing.B, cfg *envconf.Config) bool
}

I am unsure what if anything makes sense as return values, t.Run returns a bool which may fit here; additionally RunBenchmark might deserve its own issue, given the current lack of support overall for benchmarks.

Note, there is some overlap in wording with this existing function, unsure if the above naming will be confusing:

	// Run Launches the test suite from within a TestMain
	Run(*testing.M) int

The goal is to allow a more stream-lined path to writing go tests natively or otherwise with the framework of choice; don't force usage of the features package.

Potential usage:

testenv.RunTest(ctx context.Context, t *testing.T, cfg *envconf.Config) (context.Context, error){
  ...
  if err != nil {
    t.Fatal(err)
  }
  ...
   t.Run("...", func(t testing.T){
     t.Parallel()
     ...
   })

})