Test-runner: fix a bug in colored client prompt.

Colored client prompt didn't work well with
readline history feature. Remove the lame
implementation of prompt color.

test/tarantool

@@ -66,7 +66,7 @@ class Options:
         "--prompt",
         metavar = "prompt",
         dest = "prompt",
-        default = "\033[92mtarantool> \033[0m",
+        default = "tarantool> ",
         help = """Command prompt. Set to "" for no prompt. Default:
         tarantool> """)