tests/thread_flood: account for all threads already in use

[keestux]

Before there were only two: "main" and "idle". But now there can also be
a thread for "usbus". The code will now first count how many there are in
use when the tests starts. It subtracts that number from MAXTHREADS.

This resolves issue #14180 (fixes #14180)

[aabadie]

Tested on arduino-mkr1000, I confirm the test is failing on master and fixed with this PR. Thanks!
It also works the same as on master on boards without stdio over USB (tested on stm32f429i-disc1).

I have one question regarding the use of volatile, I don't think it's required here. See below. Apart from that, I think this is good.

[keestux]

There are more tests suffering from this, e.g. tests/cond_order. Perhaps we can collect corrections for all of them

[keestux]

And tests/cpp11_thread

/* http://en.cppreference.com/w/cpp/thread/thread */
int main() {
  puts("\n************ C++ thread test ***********");

  expect(sched_num_threads == 2); // main + idle

[keestux]

Can I add more fixes to this PR to solve similar problems? I have fixes for tests/cond_order, tests/mutex_order and tests/cpp11_thread.

[kaspar030]

Can I add more fixes to this PR to solve similar problems? I have fixes for tests/cond_order, tests/mutex_order and tests/cpp11_thread.

rather open individual PRs, faster to test.

BTW, #14224 also needs this!

[kaspar030]

tests/rmutex is also affected.

[kaspar030]

I used this to test: #14227

Works with dummy thread and this PR, fails with dummy thread on master.

ACK.

[kaspar030]

ACK.

@aabadie, last look?