Add PCOV support

[trejjam]

• new feature
• BC break? no

Hello,

I met an announcement about PHPUnit 8 support PCOV which should be really fast in comparison to other code coverage possibilities, e.g. https://twitter.com/kornrunner/status/1091381371447230464, https://twitter.com/RemiCollet/status/1091336903423594496, etc.

This PR adds support for it.

[milo]

@trejjam Great PR! But I have to check, why I get 0% coverage with it :)

[dg]

You should write more tests :)

[trejjam]

To make it work, I needed to configure application path in pcov.directory for my environment (probably because my test entrypoint is in different file subtree then application - which is out of scope in default/auto configuration)

[milo]

The pcov.directory should be set to a common path of all --coverage-src values.

The pcov.directory is SYSTEM,PERDIR so must be set by -d pcov.directory=... from CliTester. Any idea how to do it nice and clean?

1. Add PhpInterpreter::detectCodeCoverageEngine(): ?string and deprecate PhpInterpreter::canMeasureCodeCoverage()

2. ?

[milo]

With #400 there is easy to detect PCOV and set -d pcov.directory INI value.

[milo]

Btw. if theoretically, all code coverage engines (PHPDBG, PCOV, Xdebug) are present, which one would you prefer?

Compare https://milovo.cz/static/tester-396/

[trejjam]

I see more points of view:

• prefer Xdebug - it can cover more edge cases - like method signature, and close brace (I feel it like more robust solution)
• prefer PCOV - it is fast, lightweight, not requires Xdebug (for PHP 7.3 is still not stable)

I will prefer a fixed order of engines. For me is not important coverage value itself but changes of coverage through time. Using the same engine every time is good enough for me.

[milo]

Merged, thank you!

The reasons for preferred code coverage engines are:

1. PCOV - it is specialized extension for this purpose
2. PHPDBG - when someone runs Tester with PHPDBG, I assume that a main purpose is code coverage
3. Xdebug