Add test: ignore unavailable versions in preexisting output

[AndydeCleyre]

Before running pip-compile, the output file may contain a pinned requirement which can't be found in PyPI or whichever repo (e.g. a revoked release). This test checks that we gracefully discard these un-find-able pins, rather than cause the compile operation to fail.

The test only succeeds with the backtracking resolver, and is marked as xfail for legacy.

Related:

• Unnecessary hard failure of pip-compile when a required package is already present in output with an unavailable version #1530
• Disregard existing pins in the output which can't be found by pip, rather than failing to compile #1531, which this PR replaces

I think it would be appropriate to label this with skip-changelog.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

Thanks!