Fix iofs tests

Changes:
- embedded variables must be at the package level
- re-use embedded FS in example for tests
- bump Go module version to 1.16 for tests to run
    - This may cause issues building in 1.15

go.mod

@@ -64,4 +64,4 @@ require (
 	modernc.org/zappy v1.0.0 // indirect
 )
 
-go 1.13
+go 1.16

source/iofs/iofs_test.go

@@ -3,7 +3,6 @@
 package iofs_test
 
 import (
-	"embed"
 	"testing"
 
 	"github.com/golang-migrate/migrate/v4/source/iofs"
@@ -11,8 +10,7 @@ import (
 )
 
 func Test(t *testing.T) {
-	//go:embed testdata/migrations/*.sql
-	var fs embed.FS
+	// reuse the embed.FS set in example_test.go
 	d, err := iofs.New(fs, "testdata/migrations")
 	if err != nil {
 		t.Fatal(err)