SqlTraceFlag: Add integration tests

SqlServerDsc
- The AppVeyor configuration file was updated to include the possibility to run skip installing one or more SQL Server instances when debugging in AppVeyor to help maximize the time alloted be run.
SqlTraceFlag
- The resource is now tested with an integration tests (issue #1835).
- A new parameter ClearAllTraceFlags was added so a configuration can enforce that there should be no trace flags.
- The examples was updated to show that values should be passed as an array, even when there is only one value.
- Get-TargetResource was updated to always return an array for parameter TraceFlags, TraceFlagsToInclude, and TraceFlagsToInclude. The last two properties will always return an empty array.
- Set-TargetResource was updated to handle a single trace flag in the current state (issue #1834).
- Set-TargetResource was updated to correctly include or exclude a single flag (issue #1834).

[x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
[ ] Resource documentation updated in the resource's README.md.
[ ] Resource parameter descriptions updated in schema.mof.
[ ] Comment-based help updated, including parameter descriptions.
[ ] Localization strings updated.
[ ] Examples updated.
[x] Unit tests updated. See DSC Community Testing Guidelines.
[x] Integration tests updated (where possible). See DSC Community Testing Guidelines.
[x] Code changes adheres to DSC Community Style Guidelines.

This change is

dsccommunity / SqlServerDsc