Add failing regression test for #1973.

[frewsxcv]

#1973

failures:

---- test_homogeneous_aggregate_float_union stdout ----
thread 'test_homogeneous_aggregate_float_union' panicked at 'assertion failed: `(left == right)`
  left: `[1.0, 2.0, 3.0, 4.0]`,
 right: `[0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000029021264197102883, 0.0, 0.0, 0.0]`', src/lib.rs:268:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[frewsxcv]

Are integration tests run in CI?

[emilio]

Are integration tests run in CI?

Yes, though on x86-64 (so I think it won't trigger that issue).

This looks good to me though, thanks!