Use C++17 [[fallthrough]] in transaction_test.cc (#11663)

Summary:
(Copied from https://www.internalfb.com/diff/D46606060)

This diff makes its files safe for use with -Wimplicit-fallthrough. Now that we're using C+20 there's no reason not to use this C++17 feature to make our code safer.
It's currently possible to write code like this:
```
switch(x){
  case 1:
    foo1();
  case 2:
    foo2();
    break;
  case 3:
    foo3();
}
```
But that's scary because we don't know whether the fallthrough from case 1 was intentional or not.
The -Wimplicit-fallthrough flag will make this an error. The solution is to either  fix the bug by inserting break or indicating intention by using [[fallthrough]]; (from C++17).
```
switch(x){
  case 1:
    foo1();
    [[fallthrough]]; // Solution if we intended to fallthrough
    break;           // Solution if we did not intend to fallthrough
  case 2:
    foo2();
    break;
  case 3:
    foo3();
}
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11663

Test Plan: Existing tests

Reviewed By: jowlyzhang

Differential Revision: D47961248

Pulled By: jaykorean

fbshipit-source-id: 0d374c721bf1b328c14949dc5c17693da7311d03

utilities/transactions/transaction_test.cc

@@ -4990,7 +4990,7 @@ TEST_P(TransactionTest, DeleteRangeSupportTest) {
           }
           break;
         case WRITE_PREPARED:
-          // Intentional fall-through
+          FALLTHROUGH_INTENDED;
         case WRITE_UNPREPARED:
           if (skip_concurrency_control && skip_duplicate_key_check) {
             ASSERT_OK(s);