For testing: add rule to mock timezone.

Used by ExportImportTest.csv_export_only()

src/androidTest/java/de/dennisguse/opentracks/TimezoneRule.java

@@ -0,0 +1,36 @@
+package de.dennisguse.opentracks;
+
+import androidx.annotation.NonNull;
+
+import org.junit.rules.TestRule;
+import org.junit.runner.Description;
+import org.junit.runners.model.Statement;
+
+import java.util.TimeZone;
+
+public class TimezoneRule implements TestRule {
+
+    private final TimeZone mTimeZone;
+    private TimeZone mDeviceTimeZone;
+
+
+    public TimezoneRule(@NonNull TimeZone timeZone) {
+        mTimeZone = timeZone;
+    }
+
+    @Override
+    public Statement apply(Statement base, Description description) {
+        return new Statement() {
+            @Override
+            public void evaluate() throws Throwable {
+                try {
+                    mDeviceTimeZone = TimeZone.getDefault();
+                    TimeZone.setDefault(mTimeZone);
+                    base.evaluate();
+                } finally {
+                    TimeZone.setDefault(mDeviceTimeZone);
+                }
+            }
+        };
+    }
+}