Parameterize the iteration count used when running benchmarks, instead of...

Parameterize the iteration count used when running benchmarks, instead of hard-coded inside the test case.
Add a '-y count' option to the test driver for this purpose.  An example:

 $  ./dotest.py -v -y 25 +b -p TestDisassembly.py

...

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Collected 2 tests

1: test_run_gdb_then_lldb (TestDisassembly.DisassembleDriverMainLoop)
   Test disassembly on a large function with lldb vs. gdb. ... 
gdb benchmark: Avg: 0.226305 (Laps: 25, Total Elapsed Time: 5.657614)
lldb benchmark: Avg: 0.113864 (Laps: 25, Total Elapsed Time: 2.846606)
lldb_avg/gdb_avg: 0.503146
ok
2: test_run_lldb_then_gdb (TestDisassembly.DisassembleDriverMainLoop)
   Test disassembly on a large function with lldb vs. gdb. ... 
lldb benchmark: Avg: 0.113008 (Laps: 25, Total Elapsed Time: 2.825201)
gdb benchmark: Avg: 0.225240 (Laps: 25, Total Elapsed Time: 5.631001)
lldb_avg/gdb_avg: 0.501723
ok

----------------------------------------------------------------------
Ran 2 tests in 41.346s

OK

llvm-svn: 142598