Migrating examples to integration tests to be run by pytest

[mattkwiecien]

We currently have 9 examples we run after tests to check the code is working as expected. This code moves those examples into tests that can be run by pytest.

• Added a new tests/integration directory
• Placed tests that use subprocess.run to run the examples in this directory and marked them as integration tests
  • I prefixed set -e to ensure the test fails if any commands fail
  • I capture the stdout and print it to the terminal to mimic the previous behavior.
• Updated pytest to skip integration tests unless requested with the --integration flag.
• Updated ci.yml to run integration tests after unit tests, and appending the output to the coverage file.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (da20744) 95.4% compared to head (92ddc4f) 96.2%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #371 +/- ## ======================================== + Coverage 95.4% 96.2% +0.8% ======================================== Files 35 35 Lines 2569 2569 ======================================== + Hits 2451 2472 +21 + Misses 118 97 -21 ``` [see 3 files with indirect coverage changes](https://app.codecov.io/gh/LSSTDESC/firecrown/pull/371/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC)