Monitor mutation coverage score of test suite

[bdsl]

Broken out from #1788.

As discussed, this isn't a priority, but my unsuccessful attempt at doing this using Infection is now merged in. It isn't doing any harm beyond taking up a few minutes of build time for each commit, but obviously having a useless build step is not ideal.

This is the command used to run Infection in the current master:

psalm/.circleci/config.yml

Line 75 in a553adb

command: php -d memory_limit=4G ~/.composer/vendor/bin/infection --coverage=build/phpunit --only-covered --threads=2 || echo 'Temporarily ignoring unexplained infection failure'

. The || echo is there to suppress the non-zero return code.

The output is visible at https://circleci.com/gh/vimeo/psalm/39 and looks like:

#!/bin/bash -eo pipefail
php -d memory_limit=4G ~/.composer/vendor/bin/infection --coverage=build/phpunit --only-covered --threads=2 || echo 'Temporarily ignoring unexplained infection failure'

     ____      ____          __  _
    /  _/___  / __/__  _____/ /_(_)___  ____
    / // __ \/ /_/ _ \/ ___/ __/ / __ \/ __ \
  _/ // / / / __/  __/ /__/ /_/ / /_/ / / / /
 /___/_/ /_/_/  \___/\___/\__/_/\____/_/ /_/

Running initial test suite...

PHPUnit version: 8.2.2

Generate mutants...

Processing source code files: 543
/bin/bash: line 1:   184 Killed                  php -d memory_limit=4G ~/.composer/vendor/bin/infection --coverage=build/phpunit --only-covered --threads=2
Temporarily ignoring unexplained infection failure

I never managed to work out why Infection is apparently crashing after three minutes.

[maks-rafalko]

For reference infection/infection#705

[mundia521]

Still working out

[bdsl]

It may be possible to resolve this following the next release of Infection - I've been testing infection's master branch on Psalm. See latest comments on infection/infection#705 .

[muglug]

I think this is not worth pursuing