Fix a freeze debug log message.

[cjerdonek]

The main purpose of this PR is to fix a non-fatal logging error in a debug log message in freeze.py (first commit out of the four in this PR).

It also does the following related things:

1. (second commit) Update PipTestEnvironment to also check for logging errors when allow_stderr_error is false, and also add a test to check that this works. This will help to catch and prevent similar errors in the future.
2. (third commit) Remove a bunch of unneeded expect_stderr=True arguments in VCS tests. (If (1) and (2) had been done before, the logging error would have been detected by the test suite.)
3. (fourth and final commit of the PR) Also remove some unneeded expect_error=True in some VCS tests. This unveiled a broken test (test_git_with_non_editable_unpacking), which is also fixed in the fourth commit.

Possible future change / improvement: also check for logging errors when our tests are running even if allow_stderr_error is true.

[cjerdonek]

Hi @xavfernandez, it would be great if you could take a look at this. With the exception of a single line (the bug fix), this is a tests-only PR.

[cjerdonek]

Merging now as this is a trivial bug fix with test improvements.