test: Use terraform-modules-vmseries-tests-skeleton package

[sebastianczech]

Description

PR delivers changes after merging PR 2 with samples for Terratest test skeleton:

• in all Go files in folders examples and tests import was changed in order to use test skeleton for Terratest - github.com/PaloAltoNetworks/terraform-aws-vmseries-modules/go/testskeleton was replaced by github.com/PaloAltoNetworks/terraform-modules-vmseries-tests-skeleton/pkg/testskeleton
• Go files with folders from go/testskeleton and go/helpers were removed

Motivation and Context

Changes are part of second task from the list in comments for the issue 86.

How Has This Been Tested?

Code was tested by executing make test for examples and go test -v . in tests subfolders for modules.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.

[pimielowski]

Great work!