[Checkbox] The formula of S3 average time is wrong.

[Gabriel Zhi Chen]

Test machine: Yoda2-2
Image: sutton-wilkes-xenial-amd64-20180116-31.iso
CPU: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz (family: 0x6, model: 0x8e, stepping: 0xa)
GPU: Intel Corporation Device [8086:5917] (rev 07) (prog-if 00 [VGA controller])
BIOS: N23ET32W (1.07 )

Steps:
1. Install image sutton-wilkes-xenial-amd64-20180116-31
2. Install Plainbox and run test case 'suspend/suspend_advanced'
3. After the case executes completed, verify the average time

Expected result:
Test result should be like
Average time to enter sleep state: 0.5562 seconds
Average time to resume from sleep state: 1.6136 seconds

Actual result:
... 8< -------------------------------------------------------------------------
Calling fwts
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
==================== Test Results ====================
Average time to sleep: -3782.28306
Average time to resume: 3811.91551

Notes:
u@u-ThinkPad-X1-Carbon-6th:~$ dpkg -l | grep checkbox
ii checkbox-ng 1.1.0~ppa~ubuntu16.04.1 all CheckBoxNG test runner
ii plainbox-provider-checkbox 0.45.0~ppa~ubuntu16.04.1 amd64 CheckBox provider for PlainBox
ii python3-checkbox-ng 1.1.0~ppa~ubuntu16.04.1 all CheckBoxNG test runner (Python 3 library)
ii python3-checkbox-support 0.38.0~ppa~ubuntu16.04.1 all collection of Python modules used by PlainBox providers