Invoke-Formatter has knock on effect on subsequent call to Invoke-ScriptAnalyzer

[bergmeister]

Steps to reproduce

Using the latest state of the development branch (but also applies to RTM version of 1.17.1 and 1.16.1, therefore the bug has been present even before I joined the project).
I believe this bug is the main culprit of PSSA's tests sometimes becoming flaky and starting to fail sometimes when the test order changes.
cc @JamesWTruher

Describe "PSSA bug" {
	It "Invoke-ScriptAnalyzer works fine" {
		Invoke-ScriptAnalyzer -ScriptDefinition 'gci' | Should -Not -BeNullOrEmpty
	}

	It "Invoke-Formatter has knock on effect on Invoke-ScriptAnalyzer" {
		Invoke-Formatter 'foo'
		Invoke-ScriptAnalyzer -ScriptDefinition 'gci' | Should -Not -BeNullOrEmpty
	}

	It "Invoke-Formatter has knock on effect on Invoke-ScriptAnalyzer but it works the 2nd time" {
		Invoke-Formatter 'foo'
		Invoke-ScriptAnalyzer -ScriptDefinition 'gci'
		Invoke-ScriptAnalyzer -ScriptDefinition 'gci' | Should -Not -BeNullOrEmpty
	}
}

The 2nd test will fail, which is exposing the problem

Environment data

> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      5.1.17763.134
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.17763.134
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
> (Get-Module -ListAvailable PSScriptAnalyzer).Version | ForEach-Object { $_.ToString() }
1.17.1