Support for sub-suites

[metafates]

This PR adds support for running sub-suites from Testo suites.

Example:

type T = *testo.T

type OuterSuite struct{ testo.Suite[T] }

func (OuterSuite) Test(t T) { testo.RunSubSuite(t, new(InnerSuite)) }

type InnerSuite struct{ testo.Suite[T] }

func (InnerSuite) Test(t T) { t.Log("Hello from sub-suite!") }

How it works with plugins

New reflection field is now available:

// SuiteInfo is the information about suite.
type SuiteInfo struct {
	// Parent refers to the parent suite info.
	// Non-nil value means that current suite is sub-suite.
	Parent *SuiteInfo

	...
}

Plugins can use it. But actual test levels remain the same, levels are not shared between suites. E.g. level 1 for top level test is also 1 for sub-suite's top-level tests