pretty: update tests to use `test_config`

These tests use raw `git config` calls, which is an older style that can
cause config to bleed between tests if not manually unset. `test_config`
ensures that config is unset at the end of each test automatically.

`test_config` is chosen over `git -c` since `test_config` still ends up
calling `git config` which seems slightly more realistic to how pretty
formats would be defined normally.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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