Add tests (for pxt testdir)

tests/base/main.ts

@@ -0,0 +1 @@
+// empty - not testing anything here

tests/base/pxt.json

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+{
+    "name": "base-test",
+    "description": "Base package for tests",
+    "files": [
+        "main.ts"
+    ],
+    "public": true,
+    "dependencies": {
+        "microbit": "*",
+        "microbit-radio": "*"
+    },
+    "installedVersion": "file:."
+}

tests/hello.ts

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+basic.showString("Hello world!")