test: refactor common code into BaseTest

Refactor test code into a BaseTest class with common code. Also change
the test logic to create a separate thread for modbus test server so it
can run indepe

test: refactor common code into BaseTest

Refactor test code into a BaseTest class with common code. Also change
the test logic to create a separate thread for modbus test server so it
can run independent of client.

Tested -
```
> meson test -t 10 -C builddir/ --print-errorlogs --wrapper="valgrind --error-exitcode=1"
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
ninja: no work to do.
1/7 test_modbus_commands OK 0.73s
2/7 test_firmware OK 3.52s
3/7 test_inventory OK 5.67s
4/7 test_modbus OK 7.91s
5/7 test_events OK 9.55s
6/7 test_port OK 12.06s
7/7 test_sensors OK 16.35s

Ok: 7
Fail: 0
```

Change-Id: I7324494caf641a49f6ad345b09c0942daad0a46f
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>

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