Fix concurrency issues in the regression test b426654 (#2211)

Fix concurrency issues in the regression test b426654 (#2211)

After fixing the interruptible ranges by aligning intrinsics
inlined by Crossgen1 / 2 I have verified that the concurrency and
rooting fixes suggested by Kount finally fix the Crossgen2 build
of the test. I have made the following changes:

1) I have shortened the test to 5 seconds and I changed the add
instruction to xor so that the test cannot randomly start
overflowing again on some future faster HW.

2) I have changed the synchronization static s_timeUp to a volatile.
This probably doesn't matter on Intel but will most likely matter
a lot on ARM.

3) I have added a call to Dispose for the timeout timer to make
sure it remains rooted for the duration of the test.

Thanks

Tomas