t: add means to disable '-x' tracing for individual test scripts

t: add means to disable '-x' tracing for individual test scripts

The previous patch resolved most of the test failures caused by
running our test suite with '-x' tracing and /bin/sh, and the
following patches in this series will resolve almost all of the
remaining failures.  Unfortunately, not yet all.

Add means to disable '-x' tracing for individual test scripts by
setting the $test_untraceable variable to a non-empty value in the
test script before sourcing 'test-lib.sh'.  However, since '-x'
tracing is not an issue with recent Bash versions supporting
BASH_XTRACEFD, i.e. v4.1 and later, don't disable tracing when the
test script is run with such a Bash version even when
$test_untraceable is set.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>