There is a free option with a 20 minute build time limit(builds exceeding that time will fall). Other starter plans start from $71/mo when billed annually and all have just 1 concurrent build.
Simulating or emulating tests is good - actually testing on real hardware is better as you'll catch device (manufacturer) specific issues that would kill your app ratings in the wild.
There's no configuration files to write, nothing to download or install, nothing to really configure. Setting it up takes basically only adding your Git repository and waiting for the clone, build and test to finish.