patch 8.1.0027: difficult to make a plugin that feeds a line to a job

Problem:    Difficult to make a plugin that feeds a line to a job.
Solution:   Add the nitial code for the "prompt" buftype.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index e36089a..6a0126b 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -147,6 +147,7 @@
 	    test_perl.res \
 	    test_plus_arg_edit.res \
 	    test_preview.res \
+	    test_prompt_buffer.res \
 	    test_profile.res \
 	    test_python2.res \
 	    test_python3.res \