Make Invoke-Formatter test case assertion fail in case of incorrect casing

[alexandear]

PR Summary

This PR fixes the test "corrects case of of cmdlet inside interpolated string" for Invoke-Formatter. It follows #1885 which didn't fully fixed the test.

Additional Info

The idea is to use -BeExactly for case-sensitive strings comparison.
With changes in the PR the test fails when the expected string is '"$(get-childItem)"':

[-] UseCorrectCasing.corrects case of of cmdlet inside interpolated string 19ms (17ms|1ms)
 Expected strings to be the same, but they were different.
 String lengths are both 18.
 Strings differ at index 3.
 Expected: '"$(get-childItem)"'
 But was:  '"$(Get-ChildItem)"'
            ---^
 at Invoke-Formatter '"$(get-childitem)"' | Should -BeExactly '"$(get-childItem)"', /Users/redko.o/src/github.com/PowerShell/PSScriptAnalyzer/Tests/Rules/UseCorrectCasing.tests.ps1:14
 at <ScriptBlock>, /Users/redko.o/src/github.com/PowerShell/PSScriptAnalyzer/Tests/Rules/UseCorrectCasing.tests.ps1:14

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
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[bergmeister]

thanks for helping us catch this as well. You are totally right.
cc @kilasuit