Don't leave temporary files after running "Load" unit test

This test created temporary files but never deleted them.

Fix this by using TestFile helper class, which also simplifies the test
code.

tests/misc/garbage.cpp

@@ -12,6 +12,8 @@
 
 #include "testprec.h"
 
+#include "testfile.h"
+
 
 #include "wx/filename.h"
 #include "wx/image.h"
@@ -146,17 +148,10 @@ TEST_CASE("Load", "[garbage]")
             data[i] = rand();
 
         // write it to a file
-        wxString garbagename = wxFileName::CreateTempFileName("garbage");
-        CHECK(!garbagename.empty());
-
-        wxFile garbage(garbagename, wxFile::write);
-        CHECK(garbage.IsOpened());
-
-        CHECK(garbage.Write(data, size) == size);
-        garbage.Close();
+        TestFile tf(data, size);
 
         // try to load it by name
-        DoLoadGarbageFile(garbagename);
+        DoLoadGarbageFile(tf.GetName());
 
         // try to load it from a wxInputStream
         wxMemoryInputStream stream(data, size);