tests: Improve _scalar_check_overflow tests

[real-or-random]

At the moment, the only test is this one:

secp256k1/src/tests.c

Lines 2262 to 2267 in 1605b02

	/* Does check_overflow check catch all ones? */
	static const secp256k1_scalar overflowed = SECP256K1_SCALAR_CONST(
	0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL,
	0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL
	);
	CHECK(secp256k1_scalar_check_overflow(&overflowed));

That is, even a blank return 1 would pass the tests.

Some calls to secp256k1_scalar_check_overflow in the tests were removed in #1484 because I deemed them redundant. But that's not entirely true. Their primary purpose was to check for overflows in other test code, and this is indeed redundant. But a secondary effect of these calls was to test secp256k1_scalar_check_overflow itself.

We could revert this, but I think it's better to have a bunch of dedicated tests:

• Check a handful of static inputs including the edge cases.
• Check some random inputs. Uniformly random inputs should not overflow. But if we tweak in the right way (e.g., setting top bits), they should overflow.