Fix FrameworkDependentAppLaunch.AppHost_GlobalLocation 'Failed to restore file' CI failure

[elinor-fung]

We're hitting an intermittent failure (particularly on Debug) trying to copy over files immediately after the process exits. Since this passes the majority of the time, it does not seem to indicate an actual issue where the files are left in use.

• Increase the existing retry we have for doing this restore
• Move tests to InstallLocation test class as it fits better there (testing registered and default install locations)
• Make InstallLocation test class create one app with test behaviour enabled instead of repeatedly doing it in half of its tests cases.

This also removes explicitly running the app with the app directory as the working directory, which doesn't seem interesting for us to keep testing.

Fixes #115851

cc @dotnet/appmodel @AaronRobinsonMSFT

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[Copilot]

Pull Request Overview

This PR addresses an intermittent CI failure by increasing IO retry attempts, reorganizes and consolidates install-location tests, and removes redundant working-directory test cases.

• Increased the retry count in TestFileBackup from 5 to 25 to reduce flaky copy failures.
• Refactored InstallLocation tests to use a shared TestBehaviourEnabledApp, added two new tests for default and registered install locations, and renamed one test for clarity.
• Removed redundant working-directory variants and inlined DotNetRoot arguments in FrameworkDependentAppLaunch tests.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/installer/tests/TestUtils/TestFileBackup.cs	Increased default IO retry attempts from 5 to 25 to mitigate intermittent copy errors.
src/installer/tests/HostActivation.Tests/InstallLocation.cs	Refactored to reuse a single app instance with test behavior, added default/registered tests, and renamed a test for consistency.
src/installer/tests/HostActivation.Tests/FrameworkDependentAppLaunch.cs	Removed working-directory tests and simplified DotNetRoot usage.

Comments suppressed due to low confidence (1)

src/installer/tests/HostActivation.Tests/InstallLocation.cs:485

• The property name 'TestBehaviourEnabledApp' uses British spelling 'Behaviour' while the class 'TestOnlyProductBehavior' uses American 'Behavior'. Consider renaming to 'TestBehaviorEnabledApp' for consistency.

            public TestApp TestBehaviourEnabledApp { get; }