test: remove test-only uint256S

uint256S was previously deprecated for being unsafe. All non-test
usage has already been removed in earlier commits.

1. Tests now use uint256::FromHex() or other constructors wherever
possible without further modification.
2. Tests that can't use uint256::FromHex() because they use input
with non-hex digit characters are
  a) modified by dropping the non-hex digit characters if that
     provides useful test coverage.
  b) dropped if the test without non-hex digit characters does
     not provide useful test coverage, e.g. because it is now
     duplicated.

Additionally, use BOOST_CHECK_EQUAL where relevant on touched lines
to make error messages more readable.

src/test/fuzz/hex.cpp

@@ -37,7 +37,6 @@ FUZZ_TARGET(hex)
     if (const auto result{uint256::FromUserHex(random_hex_string)}) {
         assert(uint256::FromHex(result->ToString()));
     }
-    (void)uint256S(random_hex_string);
     try {
         (void)HexToPubKey(random_hex_string);
     } catch (const UniValue&) {

src/test/fuzz/integer.cpp

@@ -140,7 +140,7 @@ FUZZ_TARGET(integer, .init = initialize_integer)
 
     const arith_uint256 au256 = UintToArith256(u256);
     assert(ArithToUint256(au256) == u256);
-    assert(uint256S(au256.GetHex()) == u256);
+    assert(uint256::FromHex(au256.GetHex()).value() == u256);
     (void)au256.bits();
     (void)au256.GetCompact(/* fNegative= */ false);
     (void)au256.GetCompact(/* fNegative= */ true);