How can I get the full content of a PHPUnit test truncated response? I'm writing a Functional test (extending BrowserTestBase) for a Drupal REST resource.  My resource works fine in real life, but the test (running with --verbose and --debug options) throws a server error 

...resulted in a '500 500 Service unavailable (with message)' response:
The website encountered an unexpected error. Please try again later.&lt;/br&gt;&lt;/br&gt;&lt;em class="placeholder"&gt;Drupal\Component\P (truncated...)

This happens when I make the request, before I can try to catch it in an assertion.  I'm pretty sure that truncated output has the information I need to solve this, but I'm having no luck getting to it.  Drupal console docs say there's a test:run command but when I try that (v1.8.0), it tells me that's not a valid command name.  I've tried running my test through the debugger but no help there, either.

Is there any way I can get to see what's in the truncated part of the response?
My problem stemmed from trying to use GuzzleHttp to make my request, instead of BrowserTestBase::drupalGet().  I still don't know about getting to that truncated output, but using drupalGet() returns the output without truncating.