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>

api/wasm_test.go

@@ -5,7 +5,7 @@ import (
 	"math"
 	"testing"
 
-	"github.com/stretchr/testify/require"
+	"github.com/tetratelabs/wazero/internal/testing/require"
 )
 
 func TestValueTypeName(t *testing.T) {