std.format: Add unittests for %g (step 2).

[berni44]

Followup on #7810.

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[thewilsonator]

Why are so many tests comments out?

[berni44]

Why are so many tests comments out?

They don't work yet. %g isn't just selecting between %e and %f. There are other changes needed too: Mainly, precision has a different meaning and trailing zeros are removed. So the unittests are the roadmap to follow. The idea is to make them work step by step.