test: add modtime write testcase

Author: hansbogert

memfs/memory_test.go

@@ -65,6 +65,11 @@ func TestModTime(t *testing.T) {
 
 	// new calls to ModTime() retain existing mod time.
 	assert.Equal(t, modtime, fi1a.ModTime())
+
+	// Writing to to file should change the mod time
+	util.WriteFile(fs, "/file1", []byte{0}, 0o666)
+	fi1c, err := fs.Stat("/file1")
+	assert.NotEqual(t, modtime, fi1c.ModTime())
 }
 
 func TestNegativeOffsets(t *testing.T) {