Use the configured system shell in tests.

* Makefile.am (AM_TESTS_ENVIRONMENT): New variable.
* tests/redirects.org (Files opened for redirect can be executed
immediately): Use an environment variable instead of hard-coding
'/bin/sh'.

Makefile.am

@@ -121,6 +121,7 @@ override $(srcdir)/doc/version.texi:
 # Tests
 #######
 
+AM_TESTS_ENVIRONMENT = SYSTEM_SHELL='$(SHELL)'; export SYSTEM_SHELL;
 TEST_EXTENSIONS = .scm .org
 SCM_LOG_COMPILER = $(top_builddir)/pre-inst-env $(GUILE)
 AM_SCM_LOG_FLAGS = --no-auto-compile

tests/redirects.org

@@ -156,7 +156,7 @@
 :script:
 #+begin_src sh
   cat > $TEST_TMP/foo.sh <<EOF
-  #!/bin/sh
+  #!$SYSTEM_SHELL
   echo foo
   EOF
   chmod +x $TEST_TMP/foo.sh