Tests: optionally skip redirecting stdin/stdout/stderr

There is a really useful debugging technique developed by Sverre
Rabbelier that inserts "bash &&" somewhere in the test scripts, letting
the developer interact at given points with the current state.

Another debugging technique, used a lot by this here coder, is to run
certain executables via gdb by guarding a "gdb -args" call in
bin-wrappers/git.

Both techniques were disabled by 781f76b1(test-lib: redirect stdin of
tests).

Let's reinstate the ability to run an interactive shell by making the
redirection optional: setting the TEST_NO_REDIRECT environment variable
will skip the redirection.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/test-lib.sh

@@ -608,6 +608,20 @@ test_eval_ () {
 	#
 	# The test itself is run with stderr put back to &4 (so either to
 	# /dev/null, or to the original stderr if --verbose was used).
+	if test -n "$TEST_NO_REDIRECT"
+	then
+		test_eval_inner_ "$@"
+		test_eval_ret_=$?
+		if test "$trace" = t
+		then
+			set +x
+			if test "$test_eval_ret_" != 0
+			then
+				say_color error >&4 "error: last command exited with \$?=$test_eval_ret_"
+			fi
+		fi
+		return $test_eval_ret_
+	fi
 	{
 		test_eval_inner_ "$@" </dev/null >&3 2>&4
 		test_eval_ret_=$?