add tests for various actions

actions/testdata/basic.workflow

@@ -0,0 +1,21 @@
+workflow "build-and-deploy" {
+  on = "push"
+  resolves = ["deploy"]
+}
+
+action "build" {
+  uses = "./action1"
+  args = "echo 'build'"
+}
+
+action "test" {
+  uses = "docker://ubuntu:18.04"
+  args = "echo 'test'"
+  needs = ["build"]
+}
+
+action "deploy" {
+  uses = "./action2"
+  runs = ["/bin/sh", "-c", "cat $GITHUB_EVENT_PATH"]
+  needs = ["test"]
+}