fix linting issue

Author: MarkRosemaker

field_test.go

@@ -11,7 +11,7 @@ func TestErrField(t *testing.T) {
 		Field: "foo",
 		Err: &errpath.ErrField{
 			Field: "bar",
-			Err:   testErr,
+			Err:   errTest,
 		},
 	}
 	if want := `foo.bar: a test error`; err.Error() != want {

index_test.go

@@ -11,7 +11,7 @@ func TestErrIndex(t *testing.T) {
 		Index: 3,
 		Err: &errpath.ErrIndex{
 			Index: 5,
-			Err:   testErr,
+			Err:   errTest,
 		},
 	}
 	if want := `[3][5]: a test error`; err.Error() != want {

key_test.go

@@ -11,7 +11,7 @@ func TestErrKey(t *testing.T) {
 		Key: "foo",
 		Err: &errpath.ErrKey{
 			Key: "bar",
-			Err: testErr,
+			Err: errTest,
 		},
 	}
 	if want := `["foo"]["bar"]: a test error`; err.Error() != want {

write_path_test.go

@@ -7,7 +7,7 @@ import (
 	"github.com/MarkRosemaker/errpath"
 )
 
-var testErr = errors.New("a test error")
+var errTest = errors.New("a test error")
 
 func TestPath(t *testing.T) {
 	err := &errpath.ErrField{