Fix PExpire testcase

[badrishc]

Fix PExpire testcase

It was failing sporadically due to timing issues as the timeout was too low (15ms).

Example: https://github.com/microsoft/garnet/actions/runs/13042680970/job/36387756667#step:6:2873

  Error Message:
   System.InvalidOperationException : Nullable object must have a value.
  Stack Trace:
     at System.Nullable`1.get_Value()
   at Garnet.test.RespTests.KeyExpireOptionsTest(String command, Boolean testCaseSensitivity) in /_/test/Garnet.test/RespTests.cs:line 2587
   at InvokeStub_RespTests.KeyExpireOptionsTest(Object, Span`1)
   at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)