Improve integration test coverage

[edmorley]

Whilst the unit tests for this repository are very comprehensive, much of the end buildpack behaviour depends on Pack CLI / lifecycle / the buildpack API version, which aren't tested by the unit tests. For example Buildpack API 0.6 changed the default process handling in a breaking way.

Now that libcnb-test has added support for more methods of invoking the built containers (see heroku/libcnb.rs#366), we are able to add integration tests for more than just a single web process example.

This PR:

• Adds five additional integration tests.
• Drops the usage of ureq / making HTTPS requests to the web process, in favour of a simpler echo / logs output based approach. The only reason the former approach was used was because of the "valid Procfile but not valid YAML" case (see 741bbc0), however that is now tested via a simpler echo fixture that includes the offending "key: value" substring, rather than the socat example command.
• Drops usage of tempfile / manually writing files in the unit tests, in favour of reading files from the fixtures directory.
• Uses indoc! to assert against multi-line log output strings, to make the end output more clearly visible from the test, and also reduce the chance of false positives (for example, Procfile declares types -> web would previously successfully match against Procfile declares types -> web, something-else since there was no end of line marker).
• Uses the new ignore = "reason" support in Rust 1.61, to make it clearer why (integration) tests are being skipped when running cargo test.
• Remove the redundant .buildpacks(vec![BuildpackReference::Crate]) calls, since the current buildpack is the default when using libcnb-test's run_test().
• Removes RUST_BACKTRACE=1 from the Circle CI configs, since in general the backtraces are noisy (making it harder to read the actual error) and add little value over the file+line number that's already output in the standard error message. In rare scenarios where the backtrace is actually useful, it can be enabled locally, or on a Circle CI run with a debugging commit pushed to the PR etc.
• Adds heroku/builder:22 to the trusted builders list, since otherwise it slows down builds, and also means log output has additional prefixes, which mess up the multi-line assertions. Longer term we will add this builder to Pack's built-in trusted builders list, and also make libcnb-test pass --trust-builder to the pack build command:
  libcnb-test should pass --trust-builder to pack build libcnb.rs#407

GUS-W-11311966.

[Malax]

🙌🏻 This is great work!