Un-skip integration test for timestamp creation

Uses a new helper to determine that the generated timestamp is within the
bounds of the subprocess runtime. This approach could be re-used to replace
some of the `--no-stamp` tests in `taglog`.
Combine two timestamp gt/lt assertions.
Style fixes to TimestampRange helper

- New-style class declaration.
- Newline after summary in docstring.
- Rename `assert_inrange()` to `assert_in_range()`.
Convert TimestampRange into a context manager

Makes it simpler to use compared to explicitly calling `.finish()`.
Merge pull request #2 from alphagov/timestamp_integration_test

Un-skip integration test for timestamp creation