test: Catch uncaught throwable in compare.php

Example:

```
php test/compare.php test/Fixtures/lessjs-3.13.1/less/_main/
```

selectors.css has an uncaught `Error: Object could not be converted to string`
that currently stops the test instead of printing the summary.

Change-Id: I291e155a427dfbff5ce8d4c740fcbec5e16b2d6f

test/compare.php

@@ -154,6 +154,8 @@ public function handleFixture( $cssFile, $lessFile, $options ) {
 			$css = $parser->getCss();
 		} catch ( Less_Exception_Parser $e ) {
 			$css = $e->getMessage();
+		} catch ( Throwable $e ) {
+			$css = $e->__toString();
 		}
 		$css = trim( $css );