TimeZone.getPreviousTransition() should never return its input

getPreviousTransition() returns the last transition before the instant
passed into it, not the last transition before or equal to. Otherwise,
you would have to manually subtract a nanosecond in between each call if
you wanted to get a series of previous transitions.

Add similar tests for getNextTransition(), although that wasn't broken.

No change needed to the spec text or documentation, which are already
correct.

Author: ptomato

polyfill/lib/ecmascript.mjs

@@ -1902,7 +1902,7 @@ export const ES = ObjectAssign({}, ES2020, {
   },
   GetIANATimeZonePreviousTransition: (epochNanoseconds, id) => {
     const lowercap = BEFORE_FIRST_DST; // 1847-01-01T00:00:00Z
-    let rightNanos = epochNanoseconds;
+    let rightNanos = bigInt(epochNanoseconds).minus(1);
     let rightOffsetNs = ES.GetIANATimeZoneOffsetNanoseconds(rightNanos, id);
     let leftNanos = rightNanos;
     let leftOffsetNs = rightOffsetNs;

polyfill/test/timezone.mjs

@@ -209,13 +209,15 @@ describe('TimeZone', () => {
       for (let i = 0, txn = inst; i < 4; i++) {
         const transition = zone.getNextTransition(txn);
         assert(transition);
+        assert(!transition.equals(txn));
         txn = transition;
       }
     });
     it(`(${zone}).getPreviousTransition() x 4 transitions`, () => {
       for (let i = 0, txn = inst; i < 4; i++) {
         const transition = zone.getPreviousTransition(txn);
         assert(transition);
+        assert(!transition.equals(txn));
         txn = transition;
       }
     });
@@ -423,6 +425,11 @@ describe('TimeZone', () => {
       equal(nyc.getNextTransition(a1).toString(), '2019-11-03T06:00:00Z');
       equal(nyc.getNextTransition(a2).toString(), '1883-11-18T17:00:00Z');
     });
+    it('should not return the same as its input if the input is a transition point', () => {
+      const inst = Temporal.Instant.from('2019-01-01T00:00Z');
+      equal(`${nyc.getNextTransition(inst)}`, '2019-03-10T07:00:00Z');
+      equal(`${nyc.getNextTransition(nyc.getNextTransition(inst))}`, '2019-11-03T06:00:00Z');
+    });
     it('casts argument', () => {
       equal(`${nyc.getNextTransition('2019-04-16T21:01Z')}`, '2019-11-03T06:00:00Z');
     });
@@ -441,6 +448,11 @@ describe('TimeZone', () => {
       equal(london.getPreviousTransition(a1).toString(), '2020-03-29T01:00:00Z');
       equal(london.getPreviousTransition(a2).toString(), '1847-12-01T00:01:15Z');
     });
+    it('should not return the same as its input if the input is a transition point', () => {
+      const inst = Temporal.Instant.from('2020-06-01T00:00Z');
+      equal(`${london.getPreviousTransition(inst)}`, '2020-03-29T01:00:00Z');
+      equal(`${london.getPreviousTransition(london.getPreviousTransition(inst))}`, '2019-10-27T01:00:00Z');
+    });
     it('casts argument', () => {
       equal(`${london.getPreviousTransition('2020-06-11T21:01Z')}`, '2020-03-29T01:00:00Z');
     });