Reduce case iteration counts in SeekUnroll.

Reduce case iteration counts in SeekUnroll.

This test was taking too long to run on x86. After investigation, it
does not appear that there is a fundamental problem with the code
generated for the test; rather, it seems that we are running an
excessive number of iterations when using this test as a correctness
test. This change reduces the number of times we run each case from 10
to 1 unless otherwise specified on the command line.

Commit migrated from https://github.com/dotnet/coreclr/commit/13867720e00706ade79e6030f6e8441fbfdef41c