Unbreak wpilibc WritePreferencesToFile test. (#103)

ntcore recently changed GlobalDeleteAll() to not delete persistent values.
This test was relying on the old behavior to provide a clean slate.

wpilibcIntegrationTests/src/PreferencesTest.cpp

@@ -12,6 +12,7 @@
 
 #include "gtest/gtest.h"
 
+#include "ntcore.h"
 #include "Timer.h"
 
 static const char* kFileName = "networktables.ini";
@@ -59,6 +60,12 @@ TEST(PreferencesTest, ReadPreferencesFromFile) {
 TEST(PreferencesTest, WritePreferencesToFile) {
   NetworkTable::Shutdown();
   NetworkTable::GlobalDeleteAll();
+  // persistent keys don't get deleted normally, so make remaining keys
+  // non-persistent and delete them too
+  for (const auto& info : nt::GetEntryInfo("", 0)) {
+    nt::SetEntryFlags(info.name, 0);
+  }
+  NetworkTable::GlobalDeleteAll();
   std::remove(kFileName);
   NetworkTable::Initialize();
   Preferences* preferences = Preferences::GetInstance();