Improve Disk II test reliability (#195)

Before, the garbage test had only about a 92% chance of success
because it relied on random numbers not being equal.  This change just
repeats the test 5 times and allows up to 1 failure.  This increases
the chance of the test passing to a hair less that 99.8%.

test/js/cards/disk2.spec.ts

@@ -677,6 +677,9 @@ describe('DiskII', () => {
             diskII.ioSwitch(0x82);  // coil 1 off
             diskII.ioSwitch(0x8e);  // read mode
 
+            // Try this test 5 times because we could get unlucky.
+            let failures = 0;
+            for (let i = 0; i < 5; i++) {
                 // Read 5 nibbles
                 const nibbles: byte[] = [];
                 let read = false;
@@ -698,7 +701,11 @@ describe('DiskII', () => {
                 for (let i = 1; i < 5; i++) {
                     equal ||= nibbles[0] === nibbles[i];
                 }
-            expect(equal).not.toBeTruthy();
+                if (equal) {
+                    failures++;
+                }
+            }
+            expect(failures).toBeLessThan(2);
         });
 
         it('disk spins at a consistent speed', () => {