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

@@ -648,7 +648,12 @@ test_eval_ () {
 	#     be _inside_ the block to avoid polluting the "set -x" output
 	#
 
-	test_eval_inner_ "$@" </dev/null >&3 2>&4
+	if test -n "$TEST_NO_REDIRECT"
+	then
+		test_eval_inner_ "$@"
+	else
+		test_eval_inner_ "$@" </dev/null >&3 2>&4
+	fi
 	{
 		test_eval_ret_=$?
 		if want_trace