test: migrate most tests to testify

testify makes most bog-standard test checks much easier to read and
maintain, and is quite widely used. It wasn't really well known back
when go-mtree was first written, but the migration is fairly
straight-forward for most tests.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

vendor/github.com/stretchr/testify/require/require.go.tmpl

@@ -0,0 +1,6 @@
+{{ replace .Comment "assert." "require."}}
+func {{.DocInfo.Name}}(t TestingT, {{.Params}}) {
+	if h, ok := t.(tHelper); ok { h.Helper() }
+	if assert.{{.DocInfo.Name}}(t, {{.ForwardedParams}}) { return }
+	t.FailNow()
+}