tests: learn to rename files that are copied for all tests

Make files_to_copy an array of tuples in which the first element is
the source filename and the second the destination name.  That we can
rename files, useful since CR2 assumes that the trace is called trace.txt.
diff --git a/tests/utils_tests.py b/tests/utils_tests.py
index d63165e..c0aa33f 100644
--- a/tests/utils_tests.py
+++ b/tests/utils_tests.py
@@ -17,9 +17,9 @@
         self.out_dir = tempfile.mkdtemp()
         os.chdir(self.out_dir)
 
-        for fname in self.files_to_copy:
-            src_fname = os.path.join(TESTS_DIRECTORY, fname)
-            shutil.copy(src_fname, self.out_dir)
+        for src_fname, dst_fname in self.files_to_copy:
+            src_fname = os.path.join(TESTS_DIRECTORY, src_fname)
+            shutil.copy(src_fname, os.path.join(self.out_dir, dst_fname))
 
     def tearDown(self):
         os.chdir(self.previous_dir)