[lit] Improve test summary output

This change aligns the test summary output along the longest
category label.  We also properly align test counts.

Before:
```
Testing Time: 10.30s
  Unsupported Tests  : 1
  Expected Passes    : 30
```

After:
```
Testing Time: 10.29s
  Unsupported Tests:  1
  Expected Passes  : 30
```