rtu: add read status register and event support

Add support to read the status registers and generate the appropriate
events.

Tested:
Unit Test -
```
> meson test -t 10 -C builddir/ --print-errorlogs --wrapper="valgrind --error-exitcode=1" test_events
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
ninja: no work to do.
1/1 test_events        OK               9.69s

Ok:                1
Fail:              0
```

Tested on Qemu -
```
Apr 03 15:41:52 ventura phosphor-modbus-rtu[1654]: OPENBMC_MESSAGE_ID={"xyz.openbmc_project.Sensor.SensorFailure":{"SENSOR_NAME":"/xyz/openbmc_project/sensors/RPU_Coolant_Outlet_Thermometer_Status","_SOURCE":{"COLUMN":73,"FILE":"../git/common/ev
ents.cpp","FUNCTION":"sdbusplus::async::task<> phosphor::modbus::events::Events::generateSensorFailureEvent(sdbusplus::message::object_path, bool)","LINE":95,"PID":1654}}}
...
Apr 03 15:41:52 ventura phosphor-modbus-rtu[1654]: OPENBMC_MESSAGE_ID={"xyz.openbmc_project.Sensor.Threshold.ReadingCritical":{"READING_VALUE":1670.6000000000001,"SENSOR_NAME":"/xyz/openbmc_project/sensors/RPU_Coolant_Outlet_Temp_C","UNITS":"xyz
.openbmc_project.Sensor.Value.Unit.DegreesC","_SOURCE":{"COLUMN":67,"FILE":"../git/common/events.cpp","FUNCTION":"sdbusplus::async::task<> phosphor::modbus::events::Events::generateSensorReadingEvent(sdbusplus::message::object_path, phosphor::modbus::events::EventLevel, double, sdbusplus::common::xyz::openbmc_project::sensor::Value::Unit, bool)","LINE":48,"PID":1654}}}
```

Change-Id: Icd78f22cf07798d06916cc077ec3f8bfac9ee8d3
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/tests/meson.build b/tests/meson.build
index 8b1df07..c457029 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -70,6 +70,18 @@
         'test_sensors.cpp',
         'modbus_server_tester.cpp',
         device_src,
+        link_with: [modbus_common_lib],
+        dependencies: [gtest_dep, gmock_dep, default_deps, modbus_rtu_dep],
+        include_directories: ['.', common_include],
+    ),
+)
+
+test(
+    'test_events',
+    executable(
+        'test_events',
+        'test_events.cpp',
+        link_with: [modbus_common_lib],
         dependencies: [gtest_dep, gmock_dep, default_deps, modbus_rtu_dep],
         include_directories: ['.', common_include],
     ),