tests: Tidy the util functions

[real-or-random]

When I look at all the helper functions near the top of tests.c, there are still some things that could be improved:

• A lot of these functions could be moved to testutil or testrand, also functions like random_fe, random_fe_magnitude. They're currently only in tests.c but they're utils in the end.
• We have functions with group_element and field_element, but they should just be ge and fe.
• The ones in testutil should probably also get a secp256k1_testutil prefix then. You could say it's good that they stand out currently, but at least that's inconsistent with the ones in testrand, which have a prefix. Or perhaps better: we use prefixes testutil_ and testrand_, dropping the additional secp256k1_ for test functions. Then they still stand out. (The main purpose of secp256k1_ is to avoid namespace collisions if someone uses the library by just including it, but that doesn't matter in test-only code.)

Noticed when looking into #1489.