fix: testing/runner.ts and deno test (denoland/deno#4392)

After splitting "failFast" and "exitOnFail" arguments, there was a situation where failing tests did not exit with code 1.

* fixed argument value passed to Deno.runTests() in deno test
* fixed argument value passed to Deno.runTests() in testing/runner.ts
* added integration tests for deno test to ensure failFast and exitOnFail work as expected
* don't write test file to file system, but keep it in memory

testing/runner.ts

@@ -269,16 +269,15 @@ async function main(): Promise<void> {
   const exclude =
     parsedArgs.exclude != null ? (parsedArgs.exclude as string).split(",") : [];
   const allowNone = parsedArgs["allow-none"];
-  const exitOnFail = parsedArgs.failfast;
   const disableLog = parsedArgs.quiet;
 
   try {
     await runTestModules({
       include,
       exclude,
       allowNone,
-      exitOnFail,
-      disableLog
+      disableLog,
+      exitOnFail: true
     });
   } catch (error) {
     if (!disableLog) {