[committed] libstdc++: Ensure ./tzdata.zi is present in testcase

[Jonathan Wakely]

When libstdc++ is configured without an embedded copy of the tzdata.zi
file (e.g. when using --with-libstdcxx-zoneinfo) this testcase can fail.
It aborts after chrono::reload_tzdb() throws an exception due to being
unable to load tzdata.zi from the __gnu_cxx::zoneinfo_dir_override()
path used in the test.

The test passes using the default configuration because the embedded
tzdata.zi is used, and it passes if another test has already run and
created ./tzdata.zi in the test's current working directory.

The solution is to create a dummy ./tzdata.zi file, which only needs to
contain a version line because we only care that chrono::reload_tzdb()
finishes, not that it loads valid data.

No other tests are affected, because they either write out a tzdata.zi
file, or call chrono::get_tzdb() (which has a fallback to a UTC-only
database). Only calling chrono::reload_tzdb() without a file present is
a problem, as in this test.

libstdc++-v3/ChangeLog:

	* testsuite/std/time/clock/utc/leap_second_info-2.cc: Create
	empty ./tzdata.zi file.
---

Tested x86_64-linux. Pushed to trunk and gcc-16.

 .../testsuite/std/time/clock/utc/leap_second_info-2.cc         | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libstdc++-v3/testsuite/std/time/clock/utc/leap_second_info-2.cc b/libstdc++-v3/testsuite/std/time/clock/utc/leap_second_info-2.cc
index 59634b34944f..4f97c2eacaa5 100644
--- a/libstdc++-v3/testsuite/std/time/clock/utc/leap_second_info-2.cc
+++ b/libstdc++-v3/testsuite/std/time/clock/utc/leap_second_info-2.cc
@@ -103,6 +103,9 @@ Leap	2120	Dec	30	23:59:59	-	S
 
 int main()
 {
+  // chrono::reload_tzdb() requires ./tzdata.zi to be present.
+  std::ofstream("tzdata.zi") << "# version empty\n";
+
   test_known_leaps();
   test_future_leaps();
 }
-- 
2.55.0