layer: rename vars that shadowed imports

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

layer/layer_test.go

@@ -51,13 +51,13 @@ func newTestGraphDriver(t *testing.T) (graphdriver.Driver, func()) {
 		t.Fatal(err)
 	}
 
-	driver, err := newVFSGraphDriver(td)
+	graphDriver, err := newVFSGraphDriver(td)
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	return driver, func() {
-		os.RemoveAll(td)
+	return graphDriver, func() {
+		_ = os.RemoveAll(td)
 	}
 }
 
@@ -76,7 +76,7 @@ func newTestStore(t *testing.T) (Store, string, func()) {
 
 	return ls, td, func() {
 		graphcleanup()
-		os.RemoveAll(td)
+		_ = os.RemoveAll(td)
 	}
 }