[release/8.0] Backport FormsInputDateTest fixes

[ilonatommy]

Backport of #68107 and #68115 to release/8.0

Fixes #31734, Fixes #67243.

Description

The caret has to be located over the left-most segment of the 3-segment date input, otherwise SendKeys will write to wrong segment, producing non-desired date values.

The test fails deterministically before fix and passes deterministically after the fix.

Customer Impact

None, it's just a test.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

Test fix.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[Copilot]

Pull request overview

Backports servicing-branch fixes to reduce flakiness in FormsInputDateTest by making Selenium keyboard input deterministic for native date/time inputs, and un-quarantines the affected tests on release/8.0.

Changes:

• Replace direct SendKeys sequences with helper methods that (1) position the caret at the left-most segment before typing and (2) reliably clear the input.
• Remove [QuarantinedTest] annotations for the fixed tests and drop the now-unused Microsoft.AspNetCore.Testing using.
• Fix a spelling error in a comment (“Stricly” → “Strictly”).

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[dotnet-policy-service]

Hi @ilonatommy. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.