add regression test for packus_epi16 issue

[RalfJung]

#159464 got fixed without a test. This adds the missing test.

Ideally this would be tested in stdarch, but we don't have enough infrastructure for that. Testing it here is better than not testing it at all.

Cc @folkertdev

[rustbot]

r? @wesleywiser

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[RalfJung]

@bors try jobs=test-x86_64*

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling diff v0.1.13
   Compiling citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)
    Finished `dev` profile [unoptimized] target(s) in 19.90s
     Running `target/debug/citool calculate-job-matrix`
Run type: TryJob { job_patterns: Some(["test-x86_64*"]), nolimit: false }
Error: Failed to calculate job matrix

Caused by:
    Patterns `test-x86_64*` did not match any auto jobs
##[error]Process completed with exit code 1.
Post job cleanup.

[rust-bors]

💔 Test for 769811c failed: CI. Failed job:

• Calculate job matrix (web logs, enhanced plaintext logs)

[RalfJung]

@bors try jobs=x86_64*

[rust-bors]

💔 Test for 7d48998 failed: CI. Failed job:

• Calculate job matrix (web logs, enhanced plaintext logs)

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling diff v0.1.13
   Compiling citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)
    Finished `dev` profile [unoptimized] target(s) in 16.87s
     Running `target/debug/citool calculate-job-matrix`
Run type: TryJob { job_patterns: Some(["x86_64*"]), nolimit: false }
Error: Failed to calculate job matrix

Caused by:
    It is only possible to schedule up to 20 custom jobs, received 28 custom jobs expanded from 1 pattern(s)
##[error]Process completed with exit code 1.
Post job cleanup.

[RalfJung]

@bors try jobs=x86_64-gnu*

[rust-bors]

☀️ Try build successful (CI)
Build commit: 078a914 (078a914ddaaed3f5698383b5200f38b5a14a719f)
Base parent: f207aa3 (f207aa3913114c3326bc0b88acbd12b10244edf8)