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 breaks the TAP format because it is not
prefixed with a "#". Fix this.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/test-lib.sh

@@ -470,7 +470,7 @@ then
 	then
 		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
 	else
-		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		echo >&2 "# warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
 		trace=
 	fi
 fi