Given a test like:
```py
@pytest.mark.parametrize("a,b", [("b", "c"), ("c", "d")])
def test_a(a, b):
assert False
```
This modifies the diagnostic message from:
```
neotest: assert False
neotest: assert False
```
to:
```
neotest: [b-c] assert False
neotest: [c-d] assert False
```
This is the same internal value which `pytest` uses for parametrized
instances in its output, so it should be fully consistent.
Without this, the diagnostic messages aren't very useful for
parametrized tests: you can't identify _which_ of the multiple test
which execute the given line are failing.
Fixes: #30
458f35723f