tests: set a standard terminal type

This makes test-ui-color.py happy when run in a dumb terminal.

Spotted by Jesse Glick <[e-mail]>

tests/run-tests.py

@@ -1187,6 +1187,7 b' def main():'
     os.environ['http_proxy'] = ''
     os.environ['no_proxy'] = ''
     os.environ['NO_PROXY'] = ''
+    os.environ['TERM'] = 'xterm'
 
     # unset env related to hooks
     for k in os.environ.keys():