Implement tests for #Subdir

Signed-off-by: guillaume <guillaume.derouville@gmail.com>

tests/tasks/subdir/subdir_simple.cue

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+	"alpha.dagger.io/europa/dagger"
+)
+
+engine.#Plan & {
+	actions: {
+		image: engine.#Pull & {
+			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		}
+
+		mkdir: engine.#Mkdir & {
+			input: image.output
+			path:  "/test/foo"
+		}
+
+		writeChecker: engine.#WriteFile & {
+			input:       mkdir.output
+			path:        "/test/foo/hello"
+			contents:    "world"
+			permissions: 0o700
+		}
+
+		subdir: dagger.#Subdir & {
+			input: writeChecker.output
+			path:  "/test/foo"
+		}
+
+		verify: engine.#ReadFile & {
+			input: subdir.output
+			path:  "/hello"
+		} & {
+			// assert result
+			contents: "world"
+		}
+	}
+}