Make DIGRAPHS_OmitFromTests dynamic?

[wilfwilson]

Just a minor note.

@fingolfin noticed in #535 (comment):

DIGRAPHS_OmitFromTests is initialized once and then not updated. This could be fixed, e.g. by removing DIGRAPHS_OmitFromTests and modifying the code which currently uses it to directly use DIGRAPHS_IsGrapeLoaded() -- the code would arguably be simpler, too. But I don't know if you have plans to use DIGRAPHS_OmitFromTests more or have other needs for it, so I didn't do that in this PR.

In essence, Grape might be loaded sometime after Digraphs is loaded (if Digraphs is loaded with the OnlyNeeded option), so it maybe makes sense to check whether Grape is loaded each time DIGRAPHS_OmitFromTests is needed.

This won't affect our testing setup, since we don't test with the OnlyNeeded option (and even if we didn't, we don't later load Grape).

[james-d-mitchell]

I'm happy to go for this, sounds very sensible!

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