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

@@ -17,6 +17,7 @@
 
 # for git on windows so stdin will not be misdetected as attached to a
 # terminal
+test -z "$TEST_NO_REDIRECT" ||
 exec < /dev/null
 
 # Keep the original TERM for say_color
@@ -524,7 +525,12 @@ maybe_setup_valgrind () {
 test_eval_ () {
 	# This is a separate function because some tests use
 	# "return" to end a test_expect_success block early.
-	eval </dev/null >&3 2>&4 "$*"
+	if test -z "$TEST_NO_REDIRECT"
+	then
+		eval </dev/null >&3 2>&4 "$*"
+	else
+		eval "$*"
+	fi
 }
 
 test_run_ () {