Added spock test

src/test/groovy/net/fortuna/ical4j/model/WeekDaySpec.groovy

@@ -0,0 +1,20 @@
+package net.fortuna.ical4j.model
+
+import net.fortuna.ical4j.util.Dates
+import spock.lang.Specification
+import spock.lang.Unroll
+
+class WeekDaySpec extends Specification {
+
+    @Unroll
+    def 'assert result of getNegativeMonthlyOffset'() {
+
+        expect: 'returned result matches expected'
+        WeekDay.getNegativeMonthlyOffset(cal) as String == expected
+
+        where:
+        cal     | expected
+        Dates.getCalendarInstance(new Date('20180228')) | '-3WE'
+        Dates.getCalendarInstance(new Date('20191130')) | '-3WE'
+    }
+}