Support averaging power values

Support new env variables 'AVERAGE_power* = "true"' in hwmon config file.
When this env variable is set, power value is the calculated average value.
Otherwise, power value is from power*_input by default.
The new average of power is calculated since the last time the sensor's
values were changed and read.

average =
(cur_average*cur_average_interval - pre_average*pre_average_interval) /
(cur_average_interval - pre_average_interval)

hwmon config example:
AVERAGE_power2 = "true"
AVERAGE_power3 = "true"
AVERAGE_power4 = "true"

Tested: Set AVERAGE_power* in p0 OCC hwmon conf but not in p1 OCC hwmon conf,
then get power sensor info with restapi to check the values.
1. The values of p0*power are all average values.
2. The values of p1*power are all input values.

Note:
Delete $(CODE_COVERAGE_CPPFLAGS) in AM_CPPFLAGS in test/Makefile.am.
This option will define NDEBUG during configuration, then assert in
code doesn't work.

Resolves: openbmc/openbmc#3187
Signed-off-by: Carol Wang <wangkair@cn.ibm.com>
Change-Id: I8d97a7b2905c79cd4f2c276b32e7f5590ffc0483
diff --git a/test/env_unittest.cpp b/test/env_unittest.cpp
new file mode 100644
index 0000000..665f8a5
--- /dev/null
+++ b/test/env_unittest.cpp
@@ -0,0 +1,35 @@
+#include "env.hpp"
+#include "env_mock.hpp"
+#include "util.hpp"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+using ::testing::Return;
+using ::testing::StrEq;
+using ::testing::StrictMock;
+
+TEST(EnvTest, EmptyEnv)
+{
+    EXPECT_FALSE(
+        phosphor::utility::isAverageEnvSet(std::make_pair("power", "1")));
+}
+
+TEST(EnvTest, ValidAverageEnv)
+{
+    StrictMock<EnvMock> eMock;
+    envIntf = &eMock;
+
+    std::string power = "power";
+    std::string one = "1";
+    std::string two = "2";
+
+    EXPECT_CALL(eMock, getEnv(StrEq("AVERAGE"), power, one))
+        .WillOnce(Return("true"));
+    EXPECT_CALL(eMock, getEnv(StrEq("AVERAGE"), power, two))
+        .WillOnce(Return("bar"));
+
+    EXPECT_TRUE(phosphor::utility::isAverageEnvSet(std::make_pair(power, one)));
+    EXPECT_FALSE(
+        phosphor::utility::isAverageEnvSet(std::make_pair(power, two)));
+}