Read testing environment variable

Determine if an XDGMENUMAKER_TEST exists and equals "1". If it does,
read the .desktop files from the tests directory instead of reading the
standard systemwide and homedir .desktop files.

src/xdgmenumaker

@@ -314,6 +314,13 @@ def sortedcategories(applist):
 
 
 def desktopfilelist():
+    # if this env variable is set to 1, then only read .desktop files from the
+    # tests directory, not systemwide. This gives a standard set of .desktop
+    # files to compare against for testing.
+    testing = os.getenv('XDGMENUMAKER_TEST')
+    if testing == "1":
+        dirs = ['../tests']
+    else:
         dirs = []
         # some directories are mentioned twice in bd.xdg_data_dirs, once
         # with and once without a trailing /