Properly use the terminal module

The documentation for terminal says

> Changing the style is permanent even after program termination! Use the code
> `system.addQuitProc(resetAttributes)` to restore the defaults.

lib/pure/unittest.nim

@@ -209,3 +209,5 @@ if envOutLvl.len > 0:
     if $opt == envOutLvl:
       outputLevel = opt
       break
+
+system.addQuitProc(resetAttributes)