add simply library tests + fix umd and cjs mixup bug + simplify library paths

test/README.md

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+This folder is only for simple library usage tests in `CJS` and `UMD`. Real unit tests are at `src/__test__`.

test/index.js

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+const { makeProviders, makeStandardFetcher } = require('../lib/index.umd.js');
+
+const providers = makeProviders({
+  fetcher: makeStandardFetcher(fetch),
+});

test/index.mjs

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+import { makeProviders, makeStandardFetcher } from '../lib/index.mjs';
+
+const providers = makeProviders({
+  fetcher: makeStandardFetcher(fetch),
+});

test/package.json

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+{
+  "scripts": {
+    "start:umd": "node index.mjs",
+    "start:cjs": "node index.js"
+  }
+}