Explore using `insta` for build output tests

[edmorley]

This buildpack's integration tests have a number of build output assertions, since build logs are a key part of the buildpack UX.

These are currently implemented using indoc, eg:

buildpacks-python/tests/pip_test.rs

Lines 13 to 32 in bb0264c

	assert_contains!(
	context.pack_stdout,
	&formatdoc! {"
	[Determining Python version]
	No Python version specified, using the current default of Python {DEFAULT_PYTHON_VERSION}.
	To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
	[Installing Python and pip]
	Installing Python {DEFAULT_PYTHON_VERSION}
	Installing pip {PIP_VERSION}
	[Installing dependencies using pip]
	Running pip install
	Collecting typing-extensions==4.7.1 (from -r requirements.txt (line 2))
	Downloading typing_extensions-4.7.1-py3-none-any.whl.metadata (3.1 kB)
	Downloading typing_extensions-4.7.1-py3-none-any.whl (33 kB)
	Installing collected packages: typing-extensions
	Successfully installed typing-extensions-4.7.1
	"}
	);

I'd like to try out the insta crate for these instead:
https://insta.rs/