Tests: adding option to copy file into temp dir.

tests/__init__.py

@@ -1,13 +1,23 @@
 from contextlib import contextmanager
 import os
+from shutil import copy
 import tempfile
 
 
 @contextmanager
-def cdtemp():
-    """Context manager to change to/return from a tmpdir."""
+def cdtemp(files=None):
+    """Context manager to change to/return from a tmpdir.
+
+    Parameters
+    ----------
+    files : Iterable of str or Path-like
+        Set of files to copy into the temporary directory
+    """
     with tempfile.TemporaryDirectory() as tmpdir:
         cwd = os.getcwd()
+        if files:
+            for file in files:
+                copy(file, tmpdir, follow_symlinks=True)
         try:
             os.chdir(tmpdir)
             yield