test-lib: whitelist GIT_TR2_* in the environment

Add GIT_TR2_* to the whitelist of environment variables that we don't
clear when running the test suite.

This allows us to use the test suite to produce trace2 test data,
which is handy to e.g. write consumers that collate the trace data
itself.

One caveat here is that we produce trace output for not *just* the
tests, but also e.g. from this line in test-lib.sh:

    # It appears that people try to run tests without building...
    "${GIT_TEST_INSTALLED:-$GIT_BUILD_DIR}/git$X" >/dev/null
    [...]

I consider this not just OK but a feature. Let's log *all* the git
commands we're going to execute, not just those within
test_expect_*().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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