add tests for various actions

actions/testdata/pipe.workflow

@@ -0,0 +1,13 @@
+workflow "test" {
+  on = "push"
+  resolves = ["test-action"]
+}
+
+action "test-action" {
+  uses = "docker://alpine:3.9"
+  runs = ["sh", "-c", "echo $IN | grep $OUT"]
+  env = {
+    IN = "foo"
+    OUT = "foo"
+  }
+}