Parser: Pass-through PHP errors in tests (#11429)

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.

packages/block-serialization-spec-parser/shared-tests.js

@@ -77,13 +77,25 @@ const makeTest = hasPHP ? ( ...args ) => describe( ...args ) : ( ...args ) => de
 
 export const phpTester = ( name, filename ) => makeTest(
 	name,
-	'test' === process.env.NODE_ENV ? jsTester( ( doc ) => JSON.parse( require( 'child_process' ).spawnSync(
-		'php',
-		[ '-f', filename ],
-		{
-			input: doc,
-			encoding: 'utf8',
-			timeout: 30 * 1000, // abort after 30 seconds, that's too long anyway
+	'test' === process.env.NODE_ENV ? jsTester( ( doc ) => {
+		const process = require( 'child_process' ).spawnSync(
+			'php',
+			[ '-f', filename ],
+			{
+				input: doc,
+				encoding: 'utf8',
+				timeout: 30 * 1000, // abort after 30 seconds, that's too long anyway
+			}
+		);
+
+		if ( process.status !== 0 ) {
+			throw new Error( process.stderr || process.stdout );
+		}
+
+		try {
+			return JSON.parse( process.stdout );
+		} catch ( e ) {
+			throw new Error( 'failed to parse JSON:\n' + process.stdout );
 		}
-	).stdout ) ) : () => {}
+	} ) : () => {}
 );