PHP startup warnings in target PHP result in `InvalidPhpBinaryPath` exception

[asgrim]

How to reproduce:

• modify your php.ini to load an extension that does not exist, e.g. extension=blah
• try to run any PIE operation that uses that target PHP to whom the php.ini belongs to

What should happen:

• \Php\Pie\Platform\TargetPhp\PhpBinaryPath::assertValidLookingPhpBinary should not throw an exception

What actually happens:

• the exception is raised in \Php\Pie\Platform\TargetPhp\Exception\InvalidPhpBinaryPath::fromInvalidPhpBinary

Additional information:

The output from the "valid-looking PHP" check is different. We expect simply the string PHP, but we see:

Warning: PHP Startup: Unable to load dynamic library 'blah' (tried: <redacted bunch of stuff it tried>) in Unknown on line 0
PHP

Hints; would be worth checking if the warning goes to stderr; this may help us easily filter out the warning (rather than doing some kind of string-based filtering) as then we can just check stderr output is exactly the string "PHP" in the check.