tests: Resolve Coverity RESOURCE_LEAK

Commit id 'b77ce18a' added a new bitmap, but neglected to virBitmapFree it

tests/virbitmaptest.c

@@ -548,6 +548,7 @@ test10(const void *opaque ATTRIBUTE_UNUSED)
     virBitmapFree(b1);
     virBitmapFree(b2);
     virBitmapFree(b3);
+    virBitmapFree(b4);
     return ret;
 }