replace old GRIB1 loading unit tests in new-style loader

[pp-mo]

This is a bit of a late addition, but possibly there is some remaining value in the specific unit tests for GRIB1 loading
here , which we will want to somehow replicate or preserve ?
However when we retire the old GribMessage and _grib1_load_rules, all this will also cease to function since it is based on creating GribMessage objects and calling grib1_convert.

For example, there is a mock-ist test designed to check that an unrecognised PDT produces a suitable error, and similar nearby tests to ensure that we can handle various recognised and unrecognised phenomenon codings.
At least, we should check the units tests of our new code, to ensure we have covered the same issues.

Likewise, there are some complex mechanisms here to check that various types of 'timeRangeIndicator' produce various types of bounded coordinate.

Quite apart from replicating the behaviour testing here, we need to be in a position to retire a lot of the aging unittest-based code which supports this, soon if not now.