[WIP] Testing

[wxtim]

• Added some more info to the docstrings of cylc/flow/dbstatecheck.py.
• Added an xtrigger validation check for negative flow numbers.
• Added full coverage unit tests for the xtrigger's validation function [1].

[1] Additionally tested as follows (but didn't fancy adding a new dependency):

from hypothesis import given
from hypothesis.strategies import integers, text


@given(output=text(), flownum=integers(min_value=0))
def test_validate_strange(flownum, output):
    args = {'output': output, 'status': None, 'flow_num': flownum}
    validate(args)

[wxtim]

Indicates this https://github.com/cylc/cylc-flow/pull/5809/files#r1586443251 breakage

[wxtim]

converted to draft as it's parent PR branch.

[wxtim]

So far diverged from orig branch that it's not worth keeping. May pick some stuff out on reviewing target.