test: Log the output of failing subprocesses

It is not uncommon for subprocesses to fail when developing new tests,
but the default representation of CalledProcessError does not print
the stdout of the program, only that it failed.

Catch this common case when a test is about to fail, and make sure to
log any output it produced.

test/infamy/tap.py

@@ -1,5 +1,6 @@
 import contextlib
 import datetime
+import subprocess
 import sys
 import traceback
 
@@ -30,6 +31,11 @@ class Test:
 
             traceback.print_exception(e, file=self.commenter)
 
+            if type(e) is subprocess.CalledProcessError:
+                print("Failing subprocess stdout:\n", e.stdout)
+            elif len(e.args) and type(e.args[0]) is subprocess.CompletedProcess:
+                print("Failing subprocess stdout:\n", e.args[0].stdout)
+
         raise SystemExit(1)
 
     @contextlib.contextmanager