Install-DbaMaintenanceSolution: make off switches work and increase test coverage

[ReeceGoding]

Type of Change

• Bug fix (non-breaking change, fixes Install-DbaMaintenanceSolution: verification cannot be turned off, even when backing up to NUL should force us to #10133 )
• New feature (non-breaking change, adds functionality, fixes # )
• Breaking change (affects multiple commands or functionality, fixes # )
• Ran manual Pester test and has passed (Invoke-ManualPester)
• Adding code coverage to existing functionality
• Pester test is included
• If new file reference added for test, has is been added to github.com/dataplat/appveyor-lab ?
• Unit test is included
• Documentation
• Build system

Purpose

Closes #10133 and every problem I found along the way. In summary:

• Many parts of Install-DbaMaintenanceSolution can now be turned off (Verify, Checksum, Compress). We previously incorrectly claimed they could be. For example, any attempts to turn off backup verification would fail.
• Attempts to make a job that simultaneously verifies backups and writes backups to NUL now fail.
• Prevented adding the ModificationLevel parameter to the log backup job and updated the documentation to remove the false claim that this worked.
• Increased test coverage. For example, CopyOnly and ModificationLevel were not tested.
• Added an AfterEach block to the tests, so we know that the Agent Job commands that we generate are valid SQL. This helps to protect against things like typos and extra commas.

Approach

Classic TDD, which isn't something I often get to do as a SQL guy. The first commit in this PR adds a lot of tests and is by far the biggest part of this PR. If you pull only the first commit and run the tests, 8 will fail:

• Additional backup parameters all enabled.Should NOT add ModificationLevel parameter to LOG backup job
• Additional backup parameters all disabled.Should have Compress parameter set to N in backup jobs
• Additional backup parameters all disabled.Should have Verify parameter set to N in backup jobs
• Additional backup parameters all disabled.Should have CheckSum parameter set to N in backup jobs
• Additional backup parameters all but Verify disabled.Should have Compress parameter set to N in backup jobs
• Additional backup parameters all but Verify disabled.Should have CheckSum parameter set to N in backup jobs
• Backup to Nul with Verify on.Should error out and tell us our mistake
• Checksum tests when instance defaults to checksum on.Should have CheckSum parameter set to N in backup jobs

By the end of this PR, everything passes.

The scariest part of this PR is my edit to the if ($InstallJobs -and ($ChangeBackupType -or $Compress -or $CopyOnly -or $Verify -or $CheckSum -or $ModificationLevel) guard clause.

Commands to test

Install-DbaMaintenanceSolution. Test it extremely! Trust nothing I've done! I really don't want to break something this important.

Learning

This is my first time getting serious with Pester! It may not be up to standard.

[potatoqualitee]

Your first time with Pester looks great 🙌🏼 It LGTM but if anyone would like to give a second approval, pls do.

[ReeceGoding]

Thanks! I was worried that there was too much boilerplate in my tests.

Just committed a fix to a really minor issue in one of them. One of the bits was the wrong way around.

[potatoqualitee]

Thank you! Will release today.