bank-account: add ignore hint and improve HINTS.md

exercises/bank-account/HINTS.md

 
 Problems arising from running code concurrently are often intermittent because they depend on the order the code is
 executed. Therefore the last test runs many [threads](https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html) 
-several times to increase the chances of catching a bug.
+several times to increase the chances of catching a bug. That means this test should fail if your implementation is not
+[thread safe](https://en.wikipedia.org/wiki/Thread_safety), but there is a chance it will pass just because there was 
+no concurrent modification attempt. It is unlikely that this will occur several times 
+in a row since the order the code is executed should vary every time you run the test. So if you run the last test a 
+couple of times and it passes every time then you can be reasonably sure that your implementation is correct.

exercises/bank-account/src/test/java/BankAccountTest.java

         assertEquals(0, bankAccount.getBalance());
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void canDepositMoney() throws BankAccountActionInvalidException {
         bankAccount.open();
         assertEquals(10, bankAccount.getBalance());
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void canDepositMoneySequentially() throws BankAccountActionInvalidException {
         bankAccount.open();
         assertEquals(28, bankAccount.getBalance());
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void canWithdrawMoney() throws BankAccountActionInvalidException {
         bankAccount.open();
         assertEquals(5, bankAccount.getBalance());
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void canWithdrawMoneySequentially() throws BankAccountActionInvalidException {
         bankAccount.open();
         assertEquals(0, bankAccount.getBalance());
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotWithdrawMoneyFromEmptyAccount() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.withdraw(5);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotWithdrawMoreMoneyThanYouHave() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.withdraw(7);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotDepositNegativeAmount() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.deposit(-1);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotWithdrawNegativeAmount() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.withdraw(-5);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotGetBalanceOfClosedAccount() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.getBalance();
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotDepositMoneyIntoClosedAccount() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.deposit(5);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void cannotWithdrawMoneyFromClosedAccount() throws BankAccountActionInvalidException {
         bankAccount.open();
         bankAccount.withdraw(5);
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void bankAccountIsClosedBeforeItIsOpened() throws BankAccountActionInvalidException {
         expectedException.expect(BankAccountActionInvalidException.class);
         bankAccount.getBalance();
     }
 
-    @Ignore
+    @Ignore("Remove to run test")
     @Test
     public void canAdjustBalanceConcurrently() throws BankAccountActionInvalidException, InterruptedException {
         bankAccount.open();