Use `TEST_FUNCTION_FAIL` in tests

[albinahlback]

We should sort of standardize how tests fails, which I think is very suitable to do with a one call to flint_throw.

With #1646, a new error code is added for test fails, and it will also support printing FLINT types. Hence, I think that every test function should utilize flint_throw instead after this PR has been merged.

[fredrik-johansson]

Just a thought: instead of

flint_throw(FLINT_ERROR, "msg");
flint_throw(FLINT_TEST_FAIL, "msg");

we could simply have

flint_error("msg");
flint_test_fail("msg");

[albinahlback]

Sounds reasonable.

[albinahlback]

Don't use this, but rather TEST_FUNCTION_FAIL, such as

TEST_FUNCTION_FAIL(
    "Failed due to n < 10\n"
    "Got n = %{fmpz}\n",
    n);