t7005: sanitize test environment for subsequent tests

t7005: sanitize test environment for subsequent tests

Some of the editor tests manipulate the environment or config in ways
that affect future tests, but those modifications are visible to future
tests and create a footgun for them.

Use test_config, subshells, single-command environment overrides, and
test helpers to automatically undo environment and config modifications
once finished.

Best-viewed-with: --ignore-all-space
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>