[lit] Improve naming of test result categories

Improve consistency when printing test results:
Previously we were using different labels for group names (the header
for the list of, e.g., failing tests) and summary count lines.  For
example, "Failing Tests"/"Unexpected Failures".  This commit changes lit
to label things consistently.

Improve wording of labels:
When talking about individual test results, the first word in
"Unexpected Failures", "Expected Passes", and "Individual Timeouts" is
superfluous.  Some labels contain the word "Tests" and some don't.
Let's simplify the names.

Before:
```
Failing Tests (1):
  ...

Expected Passes    : 3
Unexpected Failures: 1
```

After:
```
Failed Tests (1):
  ...

Passed: 3
Failed: 1
```

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D77708