store and reset original Knob value in GetMappedRange test(release-7.2)

[flowguru]

backporting #8800

Code-Reviewer Section

The general pull request guidelines can be found here.

Please check each of the following things and check all boxes before accepting a PR.

• The PR has a description, explaining both the problem and the solution.
• The description mentions which forms of testing were done and the testing seems reasonable.
• Every function/class/actor that was touched is reasonably well documented.

For Release-Branches

If this PR is made against a release-branch, please also check the following:

• This change/bugfix is a cherry-pick from the next younger branch (younger release-branch or main if this is the youngest branch)
• There is a good reason why this PR needs to go into a release branch and this reason is documented (either in the description above or in a linked GitHub issue)

[fdb-windows-ci]

Doxense CI Report for Windows 10

• Commit ID: eb6983b
• Result: ✔️ SUCCEEDED
• Build Logs (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-cluster-tests on Linux CentOS 7

• Commit ID: eb6983b
• Duration 0:32:08
• Result: ❌ FAILED
• Error: Error while executing command: docker push ${DEV_REGISTRY}/foundationdb/foundationdb-kubernetes-sidecar:${FDB_VERSION}-${COMMIT_SHA}-1. Reason: exit status 1
• Build Logs (available for 30 days)
• Build Artifact (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-macos-m1 on macOS BigSur 11.5.2

• Commit ID: eb6983b
• Duration 0:40:40
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)
• Build Artifact (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-clang on Linux CentOS 7

• Commit ID: eb6983b
• Duration 0:40:54
• Result: ✅ SUCCEEDED
• Error: N/A
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[foundationdb-ci]

Result of foundationdb-pr on Linux CentOS 7

• Commit ID: eb6983b
• Duration 0:43:04
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)
• Build Artifact (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-macos on macOS Monterey 12.x

• Commit ID: eb6983b
• Duration 0:45:03
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)
• Build Artifact (available for 30 days)