Add missing override keyword:

* Enable the `suggest-override` warning for gcc
* Fix all functions that were flagged by that warning

src/test/core/ClosureCounter_test.cpp

@@ -313,7 +313,7 @@ class ClosureCounter_test : public beast::unit_test::suite
     }
 
 public:
-    void run()
+    void run() override
     {
         testConstruction();
         testArgs();

src/test/core/Config_test.cpp

@@ -852,7 +852,7 @@ trustthesevalidators.gov
         }
     }
 
-    void run ()
+    void run () override
     {
         testLegacy ();
         testDbPath ();

src/test/core/Coroutine_test.cpp

@@ -170,7 +170,7 @@ public:
     }
 
     void
-    run()
+    run() override
     {
         correct_order();
         incorrect_order();

src/test/core/CryptoPRNG_test.cpp

@@ -113,7 +113,7 @@ class CryptoPRNG_test : public beast::unit_test::suite
     }
 
 public:
-    void run ()
+    void run () override
     {
         testGetValues();
         testSaveLoad();

src/test/core/JobQueue_test.cpp

@@ -140,7 +140,7 @@ class JobQueue_test : public beast::unit_test::suite
     }
 
 public:
-    void run()
+    void run() override
     {
         testAddJob();
         testPostCoro();

src/test/core/SociDB_test.cpp

@@ -367,7 +367,7 @@ public:
         testSQLiteSelect ();
         testSQLiteDeleteWithSubselect();
     }
-    void run ()
+    void run () override
     {
         testSQLite ();
     }

src/test/core/Stoppable_test.cpp

@@ -187,7 +187,7 @@ class Stoppable_test
             , test_(test)
             , stop_(running)
         {}
-        ~A()
+        ~A() override
         {
             while (stop_ != stopped)
                 ;
@@ -446,7 +446,7 @@ class Stoppable_test
     };
 
 public:
-    void run()
+    void run() override
     {
         {
             Root rt(*this);

src/test/core/Workers_test.cpp

@@ -145,7 +145,7 @@ public:
         BEAST_EXPECT(cb.count.load() == 0);
     }
 
-    void run()
+    void run() override
     {
         testThreads( 0, 0,  0);
         testThreads( 1, 0,  1);