Allow dynamic setting of running tests in parallel

[jakubsob]

It'd be useful to allow running tests in parallel based on options which can be changed during R session instead of the DESCRIPTION.

• It could allow running tests in parallel locally, when we might care more about getting results faster and to run them sequentially in CI (or the other way around).
• When parallel testing is set, even when a single test file is run, e.g. with filter parameter of test_dir, a new session is spun up, leading to slower feedback from tests. It could be made easier to switch between running in parallel or sequentially.
• It would allow running tests in parallel in non-package projects.

I believe such change could be implemented by just removing the reading of DESCRIPTION file in find_parallel() and relying only on the TESTTHAT_PARALLEL variable. Then we'd have the option to switch it on and off on demand during session. It would be consistent with the ability to set number of workers with TESTTHAT_CPUS variable.