B Make WithTimeZone thread safe

Default time zone is global for a whole JVM instance, so a thread using WithTimeZone must be guaranteed that a concurrent thread do not change it in mid-execution.

Author: rzabini

approvaltests-util-tests/src/test/resources/junit-platform.properties

@@ -0,0 +1,2 @@
+junit.jupiter.execution.parallel.enabled=true
+junit.jupiter.execution.parallel.mode.default = concurrent

approvaltests/src/main/java/org/approvaltests/utils/WithTimeZone.java

@@ -1,22 +1,26 @@
 package org.approvaltests.utils;
 
 import java.util.TimeZone;
+import java.util.concurrent.locks.ReentrantLock;
 
 public class WithTimeZone implements AutoCloseable
 {
-  private final TimeZone tz;
+  private static final ReentrantLock defaultTimeZoneLock = new ReentrantLock();
+  private final TimeZone             tz;
   public WithTimeZone()
   {
     this("UTC");
   }
   public WithTimeZone(String zoneId)
   {
     tz = TimeZone.getDefault();
+    defaultTimeZoneLock.lock();
     TimeZone.setDefault(TimeZone.getTimeZone(zoneId));
   }
   @Override
   public void close()
   {
     TimeZone.setDefault(tz);
+    defaultTimeZoneLock.unlock();
   }
 }