Test ArrowTypes package in RC verification script (#379)

fixes #378
diff --git a/dev/release/verify_rc.sh b/dev/release/verify_rc.sh
index bf2103c..f38cd6b 100755
--- a/dev/release/verify_rc.sh
+++ b/dev/release/verify_rc.sh
@@ -184,6 +184,9 @@
 }
 
 test_source_distribution() {
+  pushd src/ArrowTypes
+  julia --project -e 'import Pkg; Pkg.build(); Pkg.test()'
+  popd
   julia --project -e 'import Pkg; Pkg.build(); Pkg.test()'
 }