Use --concurrency param for browser pool

[sosnovsky]

Shouldn't browser pool and concurrency always be exactly equal? Or is there a reason to have these two numbers diverge? I'd imagine, we could remove special code to choose that FLAKY-GROUP has 1 in the pool, and instead follow whatever the provided --concurrency ?

Originally posted by @tomholub in #4928 (comment)

[sosnovsky]

It seems we can remove --pool-size parameter and just use --concurrency for setting browser pool size

[tomholub]

provided it works well, yes :-)

[sosnovsky]

I think it should work well, as currently each test uses separate mock api, so they shouldn't interfere.
We can also try to increase concurrency value for flaky group, to check if they'll fail or still pass.

[sosnovsky]

Looks like we'll need to use separate values for --concurrency and --pool-size, as --concurrency param of ava means how many test files (not tests) will be executed simultaneously
(found by @IvanPizhenko at https://github.com/FlowCrypt/enterprise-server/issues/4807)