[lit] Remove a timing senstive part of `shtest-timeout.py`

[lit] Remove a timing senstive part of `shtest-timeout.py`

The `shtest-timeout.py` test was failing intermittently. It looks like
the issue is that on a resource constrained system lit is unable to run
`quick_then_slow.py` twice and print out the messages the tests expects
within the one second timeout.

The underlying issue is that the test is dependent on the performance of
the host machine is a rather fragile way. This is due to hardcoding
timeout values and having assumptions that the host machine is able to
perform a certain amount of work within the hardcoded timeout values.

We could increase the timeout values but that doesn't really fix the
underlying issue. Instead this patch removes one of fragile assumptions
in the hope that this will be enough to fix the bots.
There are other fragile assumptions in this test (e.g. `quick.py` can be
executed in less than 1 second). If the bots continue to fail we'll have
to revisit this.

rdar://problem/38774530

llvm-svn: 328702