tests/README

test: stabilize test-run-tests.t output

We have reached a point where the duration in JSON reports of
`test-run-tests.t` were greater or equal than 10 seconds, which doesn't match
anymore the regex. For example here:
https://ci.octobus.net/blue/organizations/jenkins/MercurialPy2/detail/MercurialPy2/276/pipeline

```
           "diff": "", ? (re)
-          "end": "\s*[\d\.]{4,5}", ? (re)
+          "end": "10.040",
           "result": "skip", ? (re)
```

Instead of accepting more characters, I changed the regex to accept any number
of digits before the `.` then 3 or 4 digits after. This way the regex is more
precise (only one `.` is authorized and we can ensure that the precision
doesn't change).

Differential Revision: https://phab.mercurial-scm.org/D5966

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.