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 std/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
