Freeze time in sample checks

[jaraco]

Refactored the tests to run the samples in-process instead of launching a subprocess, giving the ability to capture stdout and monkey patch other behaviors (file modified time detection).

This refactoring revealed some bugs about how the stream handling was done.

This approach gives us more control when testing the mappings.

Note that the CLI executable is still exercised in test_help*.

• Simply import the module.
• Allow args to be passed in explicitly (to facilitate testing).
• Use sys namespace.
• In test_sample, exercise the code in process.
• Fix resource warnings about unclosed source.
• Freeze time and bypass file mtime check for to avoid passing a fixed date with each command.