Isolates testify to one file, so that it is easier to remove (#460)

This starts the process of removing all dependencies from wazero, by
isolating all assertions we use into a single file. This allows us to
port those assertions as we have time, and when twitchy is gone, the
project literally has no dependencies except go!

Signed-off-by: Adrian Cole <adrian@tetrate.io>

builder_test.go

@@ -5,10 +5,9 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/stretchr/testify/require"
-
 	"github.com/tetratelabs/wazero/api"
 	"github.com/tetratelabs/wazero/internal/leb128"
+	"github.com/tetratelabs/wazero/internal/testing/require"
 	"github.com/tetratelabs/wazero/internal/u64"
 	"github.com/tetratelabs/wazero/internal/wasm"
 )