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

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