test: sensor_manager: add test and sensor_mock

Adds a mock sensor for use in future tests, as well as
adds a unit-test to sensor_manager_unittest to check that the
type of a sensor added is not validated against anything.

Change-Id: I684ec71c02f427cfa7a414a6a4162beaba2269da
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/test/sensor_mock.hpp b/test/sensor_mock.hpp
new file mode 100644
index 0000000..2b63d02
--- /dev/null
+++ b/test/sensor_mock.hpp
@@ -0,0 +1,18 @@
+#pragma once
+
+#include <gmock/gmock.h>
+
+#include "interfaces.hpp"
+#include "sensors/sensor.hpp"
+
+class SensorMock : public Sensor
+{
+    public:
+        virtual ~SensorMock() = default;
+
+        SensorMock(const std::string& name, int64_t timeout)
+            : Sensor(name, timeout) {}
+
+        MOCK_METHOD0(read, ReadReturn());
+        MOCK_METHOD1(write, void(double));
+};