patch 8.1.2341: not so easy to interrupt a script programatically

Problem:    Not so easy to interrupt a script programatically.
Solution:   Add the interrupt() function. (Yasuhiro Matsumoto, closes #2834)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 6b86b37..ac25420 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -154,6 +154,7 @@
 	test_increment \
 	test_increment_dbcs \
 	test_ins_complete \
+	test_interrupt \
 	test_job_fails \
 	test_join \
 	test_json \
@@ -361,6 +362,7 @@
 	test_increment.res \
 	test_increment_dbcs.res \
 	test_ins_complete.res \
+	test_interrupt.res \
 	test_job_fails.res \
 	test_json.res \
 	test_jumplist.res \