get rid of --serial option

[weepy]

it would be nice to mix and match serial and non serial tests - especially to not have to run them differently.

i wonder whether the function's length could be used to determine if the test was async or sync.

E.g.

serial = test.length == 1

I realise this would be a fairly significant refactor, but I think it might prove significantly more usable.

[tj]

I do arity checks for --serial to check sync vs async, but the callback when non --serial has a different behavior since it executed on exit to provide more assertions

[weepy]

which assertions ?

[tj]

in async mode the callback is used for asserting that callback(s) fired, otherwise you might get a false positive

[bergmark]

How would you decide which tests can be run in parallel with the serial tests? I can think of two ways, avoid the issue by stalling until all running branches finish when a serial test is encountered, or specify serial deps statically (across tests as well, so this might get too verbose)