test_ffi_key_set_expiry_multiple_uids fails in year 2041

[bmwiedemann]

While working on reproducible builds for openSUSE, I found that
our rnp v0.18.0 package stops passing its tests after 2041-05-13, likely because some of the test data has expiration date set there.

Test log:

[ RUN      ] rnp_tests.test_ffi_key_set_expiry_multiple_uids
/home/abuild/rpmbuild/BUILD/rnp-0.18.0-build/rnp-v0.18.0/src/tests/ffi-key-prop.cpp:125: Failure
Value of: (expired)
  Actual: true
Expected: false

[  FAILED  ] rnp_tests.test_ffi_key_set_expiry_multiple_uids (477 ms)

referring to this code:

    assert_rnp_success(rnp_locate_key(ffi, "userid", "Alice <alice@rnp>", &key));   
    expiry = 0;
    assert_rnp_success(rnp_key_get_expiration(key, &expiry));
    assert_int_equal(expiry, 674700647);
    assert_rnp_success(rnp_key_is_expired(key, &expired));
    assert_false(expired);

Background:
As part of my work on reproducible builds for openSUSE, I check that software still gives identical build results in the future.
The usual offset is +16 years, because that is how long I expect some software will be used in some places.
This showed up failing tests in our package build.
See https://reproducible-builds.org/ for why this matters.

Possible approaches are:

• mock the date
• set the expiry to the far future (e.g. year 2105, or further if the format allows it)