Factor out TempFile class and reuse it in other tests

Ensure we don't leave "mytext.dat" and "test.txt" lying around in any
directory the tests are run from by ensuring that these files are
destroyed by the test code using them.

tests/filekind/filekind.cpp

@@ -37,6 +37,8 @@
     #define fileno _fileno
 #endif
 
+#include "testfile.h"
+
 ///////////////////////////////////////////////////////////////////////////////
 // The test case
 
@@ -100,23 +102,17 @@ void FileKindTestCase::TestFd(wxFile& file, bool expected)
     CPPUNIT_ASSERT(outStream.IsSeekable() == expected);
 }
 
-struct TempFile
-{
-    ~TempFile() { if (!m_name.IsEmpty()) wxRemoveFile(m_name); }
-    wxString m_name;
-};
-
 // test with an ordinary file
 //
 void FileKindTestCase::File()
 {
     TempFile tmp; // put first
     wxFile file;
-    tmp.m_name = wxFileName::CreateTempFileName(wxT("wxft"), &file);
+    tmp.Assign(wxFileName::CreateTempFileName(wxT("wxft"), &file));
     TestFd(file, true);
     file.Close();
 
-    wxFFile ffile(tmp.m_name);
+    wxFFile ffile(tmp.GetName());
     TestFILE(ffile, true);
 }