test-lib.sh: set GIT_TRACE2_EVENT_NESTING

test-lib.sh: set GIT_TRACE2_EVENT_NESTING

The GIT_TRACE2_EVENT feed has a limited nesting depth to avoid
overloading the feed when recursing into deep paths while adding more
nested regions.

Some tests use the GIT_TRACE2_EVENT feed to look for internal events,
ensuring that intended behavior is happening.

One such example is in t4216-log-bloom.sh which looks for a statistic
given as a trace2_data_intmax() call. This test started failing under
'-x' with 2ca245f8be5 (csum-file.h: increase hashfile buffer size,
2021-05-18) because the change in stderr triggered the progress API to
create an extra trace2 region, ejecting the statistic.

This change increases the value of GIT_TRACE2_EVENT_NESTING across the
entire test suite to avoid errors like this. Future changes will remove
custom assignments of GIT_TRACE2_EVENT_NESTING from some test scripts
that were aware of this limitation.

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>