Fix printing of async test summary.

Fixes #61.

* The results from async tests are now only printed if any were actually run. 
* Additionally, the printed summary for the async test results will now only include statistics for the async tests themselves - previously, the sync and async tests were added together.

Example output without async tests:
```
Testing core-async-tutorial.core-test 

FAIL in (core-async-tutorial.core-test/fail-test) (:)
expected: false
  actual: false

Ran 3 tests containing 3 assertions.
Testing complete: 1 failures, 0 errors.
```

Example output with async tests:
```
Testing core-async-tutorial.core-test 

FAIL in (core-async-tutorial.core-test/fail-test) (:)
expected: false
  actual: false

Testing core-async-tutorial.async-test 

Ran 3 tests containing 3 assertions.
Testing complete: 1 failures, 0 errors.
Waiting on 2 asynchronous tests to complete.

Testing core-async-tutorial.async-test (async)

FAIL in (core-async-tutorial.async-test/async-fail-test) (:)
expected: false
  actual: false

Ran 2 tests containing 2 assertions.
Testing complete: 1 failures, 0 errors.
```