Remove $fileExists condition before CheckSum

[XVII]

Pull Request (PR) description

Test the CheckSum if it's provided -- allow it to override cached attribute check.

This Pull Request (PR) fixes the following issues

• Fixes #723

Task list

• [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 added/updated in README.md.
• [x] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [x] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Community Testing Guidelines.
• [x] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [x] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #724 (eb7f67d) into main (475885c) will decrease coverage by 0%. The diff coverage is n/a.

@@          Coverage Diff          @@
##           main   #724     +/-   ##
=====================================
- Coverage    72%    72%     -1%     
=====================================
  Files        30     30             
  Lines      4427   5838   +1411     
=====================================
+ Hits       3213   4209    +996     
- Misses     1214   1629    +415     

Impacted Files	Coverage Δ
.../DSCResources/DSC_xRemoteFile/DSC_xRemoteFile.psm1	89% <ø> (ø)
...ces/DSC_xPackageResource/DSC_xPackageResource.psm1	78% <0%> (-2%)	:arrow_down:
...sources/DSC_xGroupResource/DSC_xGroupResource.psm1	69% <0%> (-1%)	:arrow_down:
...xEnvironmentResource/DSC_xEnvironmentResource.psm1	91% <0%> (-1%)	:arrow_down:
...ces/DSC_xServiceResource/DSC_xServiceResource.psm1	93% <0%> (-1%)	:arrow_down:
...Resources/DSC_xUserResource/DSC_xUserResource.psm1	53% <0%> (-1%)	:arrow_down:
...urces/DSC_xScriptResource/DSC_xScriptResource.psm1	100% <0%> (ø)
...sionConfiguration/DSC_xPSSessionConfiguration.psm1	0% <0%> (ø)
...urces/DSC_xWindowsFeature/DSC_xWindowsFeature.psm1	88% <0%> (+1%)	:arrow_up:

[PlagueHO]

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