test-lib: extract Bash version check for '-x' tracing

test-lib: extract Bash version check for '-x' tracing

One of our test scripts, 't1510-repo-setup.sh' [1], still can't be
reliably run with '-x' tracing enabled, unless it's executed with a
Bash version supporting BASH_XTRACEFD (since v4.1).  We have a lengthy
condition to check the version of the shell running the test script,
and disable tracing if it's not executed with a suitable Bash version
[2].

Move this check out from the option parsing loop, so other options can
imply '-x' by setting 'trace=t', without missing this Bash version
check.

[1] 5827506928 (t1510-repo-setup: mark as untraceable with '-x',
    2018-02-24)
[2] 5fc98e79fc (t: add means to disable '-x' tracing for individual
    test scripts, 2018-02-24)

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