Get test suite to pass

[denisalevi]

This PR fixes some remaining minor issues that led to some test fails and marks tests failing for known issues as xfail, such that the test suite doesn't fail (since these tests fail expectedly).

I just ran the test suite via

import brian2cuda
brian2cuda.test(long_tests=True)

And for the first time, I got

Running Brian2CUDA version 0.post0.dev1157 from /cognition/home/denis/projects/brian2cuda/brian2cuda_repository/brian2cuda
Resetting to default preferences
Running tests in /cognition/home/denis/projects/brian2cuda/brian2cuda_repository/frozen_repos/brian2/brian2, /cognition/home/denis/projects/brian2cuda/brian2cuda_repository/brian2cuda/tests  (including long tests)
Running Brian version 2.4.2 from '/cognition/home/denis/projects/brian2cuda/brian2cuda_repository/frozen_repos/brian2/brian2'
Testing standalone

Testing standalone device "cuda_standalone"
Running standalone-compatible standard tests (single run statement)
============================= test session starts ==============================
...
= 236 passed, 1 skipped, 603 deselected, 2 xfailed, 5150 warnings in 3803.24s (1:03:23) =

Running standalone-compatible standard tests (multiple run statements)
============================= test session starts ==============================
...
=== 39 passed, 800 deselected, 3 xfailed, 330 warnings in 684.94s (0:11:24) ====

Running standalone-specific tests
============================= test session starts ==============================
...
== 89 passed, 752 deselected, 1 xfailed, 1311 warnings in 2087.32s (0:34:47) ===

OK: 3/3 test suite(s) did complete successfully.
OK: Test suit(s) for 1/1 standalone targets did complete successfully

🥳🥳🥳