Add experimental safariYieldStrategy: "time" config option

This greatly improves speed, at least in jasmine-core's own tests.

spec/core/ConfigurationSpec.js

@@ -15,7 +15,8 @@ describe('Configuration', function() {
     'seed',
     'specFilter',
     'extraItStackFrames',
-    'extraDescribeStackFrames'
+    'extraDescribeStackFrames',
+    'safariYieldStrategy'
   ];
   Object.freeze(standardBooleanKeys);
   Object.freeze(allKeys);
@@ -36,6 +37,7 @@ describe('Configuration', function() {
     expect(subject.detectLateRejectionHandling).toEqual(false);
     expect(subject.extraItStackFrames).toEqual(0);
     expect(subject.extraDescribeStackFrames).toEqual(0);
+    expect(subject.safariYieldStrategy).toEqual('count');
   });
 
   describe('copy()', function() {
@@ -137,5 +139,28 @@ describe('Configuration', function() {
       subject.update({ extraDescribeStackFrames: 100000 });
       expect(subject.extraDescribeStackFrames).toEqual(100000);
     });
+
+    it('sets safariYieldStrategy when valid', function() {
+      const subject = new privateUnderTest.Configuration();
+
+      subject.update({ safariYieldStrategy: undefined });
+      expect(subject.safariYieldStrategy).toEqual('count');
+
+      subject.update({ safariYieldStrategy: 'time' });
+      expect(subject.safariYieldStrategy).toEqual('time');
+
+      subject.update({ safariYieldStrategy: 'count' });
+      expect(subject.safariYieldStrategy).toEqual('count');
+    });
+
+    it('rejcts invalid safariYieldStrategy values', function() {
+      const subject = new privateUnderTest.Configuration();
+
+      expect(function() {
+        subject.update({ safariYieldStrategy: 'thyme' });
+      }).toThrowError(
+        "Invalid safariYieldStrategy value. Valid values are 'count' and 'time'."
+      );
+    });
   });
 });