PromQL: Add tests for time/date funcs with arg

Add tests for the date and time functions where an argument is
specified.

Suggested by @grobie:
https://github.com/prometheus/prometheus/pull/1984#issuecomment-246508286

`1136239445` is the reference time used by Go:
https://golang.org/src/time/format.go

promql/testdata/functions.test

@@ -370,22 +370,40 @@ clear
 eval instant at 0m year()
   {} 1970
 
+eval instant at 0m year(vector(1136239445))
+  {} 2006
+
 eval instant at 0m month()
   {} 1
 
+eval instant at 0m month(vector(1136239445))
+  {} 1
+
 eval instant at 0m day_of_month()
   {} 1
 
+eval instant at 0m day_of_month(vector(1136239445))
+  {} 2
+
 # Thursday.
 eval instant at 0m day_of_week()
   {} 4
 
+eval instant at 0m day_of_week(vector(1136239445))
+  {} 1
+
 eval instant at 0m hour()
   {} 0
 
+eval instant at 0m hour(vector(1136239445))
+  {} 22
+
 eval instant at 0m minute()
   {} 0
 
+eval instant at 0m minute(vector(1136239445))
+  {} 4
+
 # 2008-12-31 23:59:59 just before leap second.
 eval instant at 0m year(vector(1230767999))
   {} 2008