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/assert/doc.go

@@ -0,0 +1,50 @@
+// Package assert provides a set of comprehensive testing tools for use with the normal Go testing system.
+//
+// # Note
+//
+// All functions in this package return a bool value indicating whether the assertion has passed.
+//
+// # Example Usage
+//
+// The following is a complete example using assert in a standard test function:
+//
+//	import (
+//	  "testing"
+//	  "github.com/stretchr/testify/assert"
+//	)
+//
+//	func TestSomething(t *testing.T) {
+//
+//	  var a string = "Hello"
+//	  var b string = "Hello"
+//
+//	  assert.Equal(t, a, b, "The two words should be the same.")
+//
+//	}
+//
+// if you assert many times, use the format below:
+//
+//	import (
+//	  "testing"
+//	  "github.com/stretchr/testify/assert"
+//	)
+//
+//	func TestSomething(t *testing.T) {
+//	  assert := assert.New(t)
+//
+//	  var a string = "Hello"
+//	  var b string = "Hello"
+//
+//	  assert.Equal(a, b, "The two words should be the same.")
+//	}
+//
+// # Assertions
+//
+// Assertions allow you to easily write test code, and are global funcs in the `assert` package.
+// All assertion functions take, as the first argument, the `*testing.T` object provided by the
+// testing framework. This allows the assertion funcs to write the failings and other details to
+// the correct place.
+//
+// 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 assert