regulators: Add phase fault type to parser

Enhance the JSON configuration file parser to support the new
phase fault type values.

Add gtest test cases to exercise new code.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: If5ac011f740fdb6f24692c713a56d4947709d660
diff --git a/phosphor-regulators/src/config_file_parser.cpp b/phosphor-regulators/src/config_file_parser.cpp
index be28fdc..5330f59 100644
--- a/phosphor-regulators/src/config_file_parser.cpp
+++ b/phosphor-regulators/src/config_file_parser.cpp
@@ -738,6 +738,27 @@
     return std::make_unique<OrAction>(std::move(actions));
 }
 
+PhaseFaultType parsePhaseFaultType(const json& element)
+{
+    std::string value = parseString(element);
+    PhaseFaultType type{};
+
+    if (value == "n")
+    {
+        type = PhaseFaultType::n;
+    }
+    else if (value == "n+1")
+    {
+        type = PhaseFaultType::n_plus_1;
+    }
+    else
+    {
+        throw std::invalid_argument{"Element is not a phase fault type"};
+    }
+
+    return type;
+}
+
 std::unique_ptr<PMBusReadSensorAction> parsePMBusReadSensor(const json& element)
 {
     verifyIsObject(element);
@@ -1054,11 +1075,7 @@
 
 SensorType parseSensorType(const json& element)
 {
-    if (!element.is_string())
-    {
-        throw std::invalid_argument{"Element is not a string"};
-    }
-    std::string value = element.get<std::string>();
+    std::string value = parseString(element);
     SensorType type{};
 
     if (value == "iout")