t/test-lib: fix TAP format for BASH_XTRACEFD warning

When the Bash version is too old to support BASH_XTRACEFD we print a
warning to stderr. This warning is not prefixed with "#", which causes
TAP parsers to (wrongly) interpret the warning as part of the protocol.

Fix this issue by prefixing the warning with a "#" so that it is treated
as comment.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>