[helm] Fix .helmignore to not package tests

[affo]

Purpose

Linked issue: close #xxx

No issue created.

We were inadvertently packaging Helm tests as part of the charts.

Brief change log

Add tests as part of .helmignore

Tests

Manual test:

With the change:

helm package .
Successfully packaged chart and saved it to: /Users/lorenzo/repos/fluss/helm/fluss-0.10.0-incubating.tgz
➜  helm git:(la-fix-helmignore) ✗ tar tf ./fluss-0.10.0-incubating.tgz| grep tests
# nothing

Without

git stash
Saved working directory and index state WIP on la-fix-helmignore: 2f8cadad [fluss-client] Support Complex Data Types on the Java Typed API (Map and Array) (#2814)
➜  helm git:(la-fix-helmignore) ✗ helm package .                                  
Successfully packaged chart and saved it to: /Users/lorenzo/repos/fluss/helm/fluss-0.10.0-incubating.tgz
➜  helm git:(la-fix-helmignore) ✗ tar tf ./fluss-0.10.0-incubating.tgz| grep tests
fluss/tests/imagepullsecrets_test.yaml

API and Format

Documentation

[morazow]

Looks good, thanks!

[affo]

@xx789633 would you be willing to review this one?
Is there a way to re-run CI?
Probably flakey