Parser: Pass-through PHP errors in tests

[dmsnell]

In #11320 we started running all parsers against the same set of unit
tests. Unfortunately when the PHP-based parsers failed inside the PHP
process or returned non-JSON data then the tests would fail without
providing any information about why.

My personal workaround was to manually run the PHP test runner from
the command line to see the output. This was inefficient.

In this patch we're trapping the response code from the PHP test runner
and throwing an Error with the output from php so that our jest
tests can see and report them. This will make it easier debug failing
tests.

Testing

You can see the impact of the change by introducing a syntax error in the
default PHP parser. Inside the parse() method I added a new line with
<!-- as its contents - invalid PHP.

In master I ran: npm run test-unit packages/block-serializer-default-parser

 ● block-serialization-default-parser-php › generic tests › should grab HTML soup after blocks

    SyntaxError: Unexpected token P in JSON at position 1
        at JSON.parse (<anonymous>)

In this branch though…

  ● block-serialization-default-parser-php › generic tests › should grab HTML soup after blocks


    Parse error: syntax error, unexpected '<' in /Users/dmsnell/PhpstormProjects/gutenberg/packages/block-serialization-default-parser/parser.php on line 183