Adjust TestConcurrentEvents.py test running for Linux.

Related to this bug:
http://llvm.org/bugs/show_bug.cgi?id=16714

On TOT lldb svn r202507, I found several of the tests disabled within
TestConcurrentEvents.py to run fine, and several of them to fail 100%
of the time on my system (25 out of 25 times).

This changes the following tests for Linux:

Enables (these all work consistently):
* test_many_watchpoints_dwarf
* test_signal_watch_break_dwarf (line 250)
* test_signal_watch_break_dwarf (line 260 - same named test?)
* test_crash_with_watchpoint_dwarf
* test_crash_with_watchpoint_breakpoint_signal_dwarf
* test_delayed_crash_with_breakpoint_watchpoint_dwarf

Marks as expected failure (these all fail consistently):
* test_many_watchpoints_dwarf
* test_watch_break_dwarf
* test_delay_watch_break_dwarf
* test_watch_break_dwarf_delay
* test_signal_watch_dwarf
* test_delay_signal_watch_dwarf
* test_signal_delay_watch_dwarf
* test_two_breakpoints_one_watchpoint_dwarf
* test_breakpoints_delayed_breakpoint_one_watchpoint_dwarf
* test_two_watchpoint_threads_dwarf
* test_watchpoint_with_delay_watchpoint_threads_dwarf
* test_two_watchpoints_one_breakpoint_dwarf
* test_two_watchpoints_one_delay_breakpoint_dwarf
* test_watchpoint_delay_watchpoint_one_breakpoint_dwarf
* test_two_watchpoints_one_signal_dwarf
* test_signal_watch_break_dwarf