test: physical: Fix cppcoreguidelines-explicit-virtual-functions

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../test/physical.cpp:51:13: error: prefer using 'override' or (rarely) 'final' instead of 'virtual' [cppcoreguidelines-explicit-virtual-functions,modernize-use-override,-warnings-as-errors]
    virtual ~MockLed()
    ~~~~~~~~^
                       override
```

Change-Id: If8fa33778605f485991723c24c3568d1c1af1cbb
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/test/physical.cpp b/test/physical.cpp
index 8ecef90..7656cb2 100644
--- a/test/physical.cpp
+++ b/test/physical.cpp
@@ -52,7 +52,7 @@
     MockLed& operator=(MockLed& other) = delete;
     MockLed&& operator=(MockLed&& other) = delete;
 
-    virtual ~MockLed()
+    ~MockLed() override
     {
         chmod(root.c_str(), S_IRUSR | S_IWUSR | S_IXUSR);
         fs::remove_all(root);