[CI] Added testing framework

[liranbg]

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove irrelevant fields.]

• Chart Version bumped
• Title of the PR starts with chart name (e.g. [mychartname])

Description

Why:

What:

• Improve chart validation by integrating tests into the CI process
  • Added generic test framework (tests/test_runner.sh, tests/lib/common.sh) for discoverable chart tests
  • Added example test for mlrun chart (MySQL image tag validation)
• Upgraded GitHub Actions: actions/checkout@v6, Helm setup action, Minikube v1.37.0

How to run locally:

# Run all chart tests
make test-charts

# Run a specific chart test
./tests/mlrun/test_mysql_tag.sh