[subset] GPOS4 MarkBase subsetting support

test/subset/data/tests/layout.gpos4.tests

diff --git a/test/subset/data/tests/layout.gpos4.tests b/test/subset/data/tests/layout.gpos4.tests
new file mode 100644
index 0000000..fd2b688
--- /dev/null
+++ b/test/subset/data/tests/layout.gpos4.tests
@@ -0,0 +1,18 @@
+FONTS:
+gpos4_multiple_anchors_1.otf
+
+PROFILES:
+keep-layout.txt
+keep-layout-retain-gids.txt
+
+SUBSETS:
+A
+AB
+AC
+ABC
+ACE
+ABCE
+ACD
+ACDEF
+ABCD
+*