Add PCOV_ENABLED env var as enable/disable switch

[Slamdunk]

Hi, xDebug:v3 has introduced the XDEBUG_MODE environment variable to turn xDebug on and off.

In context where you spawn child processes like ParaTest env vars are easier to handle and propagate to external procs than custom ini settings passed as options to the php-cli binary (i.e. php -d pcov.enabled=1).

There is no standardized way to propagate ini options, so now we need to explicitly tell them like:

php -d pcov.enabled=1 vendor/bin/paratest --passthru-php="'-d' 'pcov.enabled=1'"

Env vars on the contrary are simple to propagate, so in the context of xDebug it's easier:

XDEBUG_MODE=coverage vendor/bin/paratest

Would it be possible to introduce a PCOV_ENABLED env var to enable/disable pcov?