Release SIG_ALRM handler before start a new test.

To prevent SIG_ALRM and its handler from previous test interrupts next test case,
added reset SIG_ALRM before start of each tests, just before pool_wide_cleanup().
Added an option to exclude some test sets from selected mode.

Now, user can exclude some set of tests by specifying set.

Available sets:
OVS: All openvswitch network tests
BRIDGE: All bridge network tests
OPS: All VM operation tests
LSTOR: All local storage tests
CPU: All CPU tests
Merge pull request #60 from bpwook/alarmclock

Release SIG_ALRM at the start of each tests.
Merge pull request #59 from bpwook/opt_backend

Added an option to exclude some test sets from selected mode.
Added "skip" as a result of test case.

Now a method can be skippable. Skipped test is not considered as failure.
Make GRO test skippable.

If GRO offload is fixed, it is pointless running GRO tests.