fix: add global test registry (#568)

After adding deno test command a new problem appeared.

If you try running deno test inside this repo test runner will find 
80 test files but won't run any tests! This is caused by fact that 
deno test uses tagged version of standard library which causes 
test function available inside to repo to be different function that 
test available in standard lib used by deno test.

testing/mod.ts

@@ -69,8 +69,29 @@ function print(txt: string, newline: boolean = true): void {
   Deno.stdout.writeSync(encoder.encode(`${txt}`));
 }
 
+declare global {
+  interface Window {
+    /**
+     * A global property to collect all registered test cases.
+     *
+     * It is required because user's code can import multiple versions
+     * of `testing` module.
+     *
+     * If test cases aren't registered in a globally shared
+     * object, then imports from different versions would register test cases
+     * to registry from it's respective version of `testing` module.
+     */
+    __DENO_TEST_REGISTRY: TestDefinition[];
+  }
+}
+
+let candidates: TestDefinition[] = [];
+if (window["__DENO_TEST_REGISTRY"]) {
+  candidates = window.__DENO_TEST_REGISTRY as TestDefinition[];
+} else {
+  window["__DENO_TEST_REGISTRY"] = candidates;
+}
 let filterRegExp: RegExp | null;
-const candidates: TestDefinition[] = [];
 let filtered = 0;
 
 // Must be called before any test() that needs to be filtered.