patch 8.1.1305: there is no easy way to manipulate environment variables

Problem:    There is no easy way to manipulate environment variables.
Solution:   Add environ(), getenv() and setenv(). (Yasuhiro Matsumoto,
            closes #2875)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 6b966e6..1f50bd8 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -104,6 +104,7 @@
 	test_erasebackword \
 	test_escaped_glob \
 	test_eval_stuff \
+	test_environ \
 	test_ex_equal \
 	test_ex_undo \
 	test_ex_z \
@@ -320,6 +321,7 @@
 	test_digraph.res \
 	test_display.res \
 	test_edit.res \
+	test_environ.res \
 	test_erasebackword.res \
 	test_escaped_glob.res \
 	test_eval_stuff.res \