regulators: Clear phase fault history in Device

Enhance the clearErrorHistory() method of the Device class to clear
phase fault error history.

Update the gtest test case to exercise the modified code.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I109932c0f4812bb36af05ac24e47a561270b4fcc
diff --git a/phosphor-regulators/src/device.cpp b/phosphor-regulators/src/device.cpp
index bde41cc..7d80851 100644
--- a/phosphor-regulators/src/device.cpp
+++ b/phosphor-regulators/src/device.cpp
@@ -50,6 +50,12 @@
 
 void Device::clearErrorHistory()
 {
+    // Clear error history in phase fault detection, if defined
+    if (phaseFaultDetection)
+    {
+        phaseFaultDetection->clearErrorHistory();
+    }
+
     // Clear error history in each rail
     for (std::unique_ptr<Rail>& rail : rails)
     {