[lldb/test] Relax NSDate mock test for non-Apple platforms

On Ubuntu, a formatted date prints as "GMT" instead of "UTC", which is
ok.

http://lab.llvm.org:8011/builders/lldb-aarch64-ubuntu/builds/4520/steps/test/logs/stdio

lldb/unittests/DataFormatter/MockTests.cpp

@@ -9,6 +9,7 @@
 #include "lldb/DataFormatters/Mock.h"
 #include "lldb/Utility/StreamString.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringRef.h"
 #include "gtest/gtest.h"
 #include <string>
 
@@ -36,5 +37,6 @@ TEST(DataFormatterMockTest, NSDate) {
   EXPECT_EQ(formatDateValue(std::numeric_limits<time_t>::max()), llvm::None);
   EXPECT_EQ(formatDateValue(std::numeric_limits<time_t>::min()), llvm::None);
 
-  EXPECT_EQ(*formatDateValue(0), "2001-01-01 00:00:00 UTC");
+  EXPECT_TRUE(
+      llvm::StringRef(*formatDateValue(0)).startswith("2001-01-01 00:00:00"));
 }