Ensure stdout and stderr from failed test runs are reported from run_tests.py.

[chinmaygarde]

We print stdout and stderr only from successful test or benchmark runs. Because
of the raised exception, a non-zero exit code would gobble up output from these
runs. This is actually the situation where this information would be most
useful. Print all the information we have before raising the exception.

[chinmaygarde]

Ran into this while debugging the presubmit failures in #25749. Because of the silent failure, I mistook the failure for a flake. Now, the output would be:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Failed Command:

/Users/chinmaygarde/VersionControlled/engine/src/out/host_release/flutter_tester --force-multithreading --disable-observatory --use-test-fonts /Users/chinmaygarde/VersionControlled/engine/src/out/paragraph_builder_test.dart.kernel.dill

Exit Code: -6

STDERR: 
[FATAL:flutter/runtime/dart_vm_initializer.cc(22)] Error while initializing the Dart VM: Precompiled runtime requires a precompiled snapshot

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!