patch 8.2.0243: insufficient code coverage for ex_docmd.c functions

Problem:    Insufficient code coverage for ex_docmd.c functions.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5618)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 40e56a2..3a4979b 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -60,6 +60,7 @@
 	test_blob \
 	test_blockedit \
 	test_breakindent \
+	test_buffer \
 	test_bufline \
 	test_bufwintabinfo \
 	test_cd \
@@ -305,6 +306,7 @@
 	test_blob.res \
 	test_blockedit.res \
 	test_breakindent.res \
+	test_buffer.res \
 	test_bufline.res \
 	test_bufwintabinfo.res \
 	test_cd.res \