fix: Kestrel Http3 LoadDevelopmentCertificateViaConfiguration test stabilization

[DeagleGross]

Spotted test failing multiple times in an unrelated to Http3 \ certs PR change:
#65186

Test uses the hardcoded path to save\load the certificate:

private static string GetCertificatePath()
{
    var appData = Environment.GetEnvironmentVariable("APPDATA");
    var home = Environment.GetEnvironmentVariable("HOME");
    var basePath = appData != null ? Path.Combine(appData, "ASP.NET", "https") : null;
    basePath = basePath ?? (home != null ? Path.Combine(home, ".aspnet", "https") : null);
    return Path.Combine(basePath, $"{typeof(Http3TlsTests).Assembly.GetName().Name}.pfx");
}

and the guess is that it is corrupted between the runs. I added saving and deleting the cert after test is finished to ensure there is no effect on this between the runs. If that does not help, we should quarantine it and fix later

Fixes #65248

[Copilot]

Pull request overview

This PR addresses test flakiness in the LoadDevelopmentCertificateViaConfiguration test by ensuring proper cleanup of certificate files between test runs. The test was failing intermittently due to corrupted certificate files persisting across test executions.

Changes:

• Wrapped the test logic in a try-finally block to guarantee certificate file cleanup
• Moved certificate path retrieval outside the try block for proper scoping
• Added file deletion in the finally block to prevent file corruption between test runs