time.mktime_EOVERFLOW: set an explicit timezone.

Bug: http://b/283397453
Test: export TZ=Asia/Shanghai
Change-Id: Ia03822adad367abf35a0fdf55d7367cd1764fcdd
diff --git a/tests/time_test.cpp b/tests/time_test.cpp
index d16600c..f89fa9a 100644
--- a/tests/time_test.cpp
+++ b/tests/time_test.cpp
@@ -168,6 +168,8 @@
 }
 
 TEST(time, mktime_EOVERFLOW) {
+  setenv("TZ", "UTC", 1);
+
   struct tm t;
   memset(&t, 0, sizeof(tm));