Make validation result comparison test messages more helpful

Validation results comparison assertions (ValidationTestTrait::assertValidationResultsEqual) do not currently provide enough detail to be valuable or actionable, returning failure messages like this:

Failed asserting that actual size 2 matches expected size 1.

This issue will replace the above with this, for example:

Failed asserting that two arrays are identical.
--- Expected
+++ Actual
@@ @@
         )
         'summary' => 'There is not enough disk space to
create a staging area.'
     )
+    1 => Array &3 (
+        'severity' => 1
+        'messages' => Array &4 (
+            0 => 'Xdebug is enabled, which may have a negative
performance impact on Package Manager and any modules that use it.'
+        )
+        'summary' => null
+    )
 )

Array diffs aren't pretty on the command line, but the important details are all there. The improvement in PhpStorm is stark: