test: add test for events with device sensors

Add test cases to test events with device sensors for threshold events
and sensor failures.

Tested:
```
> meson test -t 10 -C builddir/ --print-errorlo

test: add test for events with device sensors

Add test cases to test events with device sensors for threshold events
and sensor failures.

Tested:
```
> meson test -t 10 -C builddir/ --print-errorlogs --wrapper="valgrind --error-exitcode=1" test_device_events
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
[2/2] Linking target tests/test_device_events
1/1 test_device_events OK 12.47s

Ok: 1
Fail: 0
```

Change-Id: Ic4b70f107eba2e96ce3235f6364b03a88a73c5d7
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>

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