Migrate schema tests to helm unittests

When the values schema for the umbrella Big Bang chart was added, we also added a number of tests to try to avoid regressions and increase confidence when making changes.

Those tests used a very primitive combination of bats and helm template commands to test valid and invalid values being passed in to the Big Bang chart.

With the addition of https://github.com/helm-unittest/helm-unittest we are able to accomplish the same thing with a more conventional set of tools.

We should migrate the existing tests over to helm unittests with the benefits being:

• consistent test framework
• tests would now be run as part of CI (the bats schema tests are not run as part of CI)

An example of a test asserting the schema fails to validate with the given values - https://github.com/helm-unittest/helm-unittest/blob/main/test/data/v3/with-schema/tests/schema-validation_test.yaml