Added more tests

src/build/fs/Walk_test.go

@@ -19,7 +19,12 @@ func TestWalk(t *testing.T) {
 	assert.Contains(t, files, "Walk_test.go")
 }
 
-func TestNonExisting(t *testing.T) {
+func TestWalkFile(t *testing.T) {
+	err := fs.Walk("Walk.go", func(file string) {})
+	assert.NotNil(t, err)
+}
+
+func TestWalkNonExisting(t *testing.T) {
 	err := fs.Walk("does-not-exist", func(file string) {})
 	assert.NotNil(t, err)
 }