Use temp .netrc file for integration tests and support NETRC environment variable

[chuckwondo]

This primary intent for the PR was to address #806. Because the changes required for #806 also produced at least partial completion of #743 and #480, it also includes the full set of changes for them as well. All 3 issues are very closely related, so it didn't make much sense to attempt to separate them out.

As a result,

• contributors may now run integration tests locally without clobbering their existing ~/.netrc file, if they have one
• because the bullet above required a means for configuring integration tests to use a temporary .netrc file location, it made sense to do so by setting a NETRC environment variable for this purpose, so users may now also set this environment variable, if they need to specify a different location for their .netrc file
• to better enable local execution of integration tests, some refactoring was required, which led to simply folding in the changes required to fix Update integration tests to use pytest conventions #743 as well

Fixes #806
Fixes #743
Fixes #480

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📚 Documentation preview 📚: https://earthaccess--808.org.readthedocs.build/en/808/

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