Add logging levels for unit tests

Add logging levels for the mock environment for unit tests. Values
map to a string name instead of an integer like in the normal
environment.

spec/env_mock.lua

@@ -106,6 +106,15 @@ local smr_mock_env = {
 	end),
 	http_response_cookie = spy.new(function(req,name,value) req.cookies = {[name] = value} end),
 	log = spy.new(function(priority, message) --[[print(string.format("[LOG %q]: %s",priority,message))]] end),
+	--Logging:
+	LOG_DEBUG = "debug",
+	LOG_INFO = "info",
+	LOG_NOTICE = "notice",
+	LOG_WARNING = "warning",
+	LOG_ERR = "error",
+	LOG_CRIT = "critical",
+	LOG_ALERT = "alert",
+	LOG_EMERG = "emergency",
 	sha3 = spy.new(function(message) return "digest" end),
 }