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/doc.go

@@ -0,0 +1,31 @@
+// Package require implements the same assertions as the `assert` package but
+// stops test execution when a test fails.
+//
+// # Example Usage
+//
+// The following is a complete example using require in a standard test function:
+//
+//	import (
+//	  "testing"
+//	  "github.com/stretchr/testify/require"
+//	)
+//
+//	func TestSomething(t *testing.T) {
+//
+//	  var a string = "Hello"
+//	  var b string = "Hello"
+//
+//	  require.Equal(t, a, b, "The two words should be the same.")
+//
+//	}
+//
+// # Assertions
+//
+// The `require` package have same global functions as in the `assert` package,
+// but instead of returning a boolean result they call `t.FailNow()`.
+// A consequence of this is that it must be called from the goroutine running
+// the test function, not from other goroutines created during the test.
+//
+// Every assertion function also takes an optional string message as the final argument,
+// allowing custom error messages to be appended to the message the assertion method outputs.
+package require