[Dates] Make test more robust against non-UTC timezones (#55829)

`%M` is the format specifier for the minutes, not the month (which
should be `%m`), and it was used twice.

Also, on macOS `Libc.strptime` internally calls `mktime` which depends
on the local timezone. We now temporarily set `TZ=UTC` to avoid
depending on the local timezone.

Fix #55827.

(cherry picked from commit d6fa66f)

stdlib/Dates/test/types.jl

@@ -263,7 +263,11 @@ end
 end
 
 @testset "issue #31524" begin
-    dt1 = Libc.strptime("%Y-%M-%dT%H:%M:%SZ", "2018-11-16T10:26:14Z")
+    # Ensure the result doesn't depend on local timezone, especially on macOS
+    # where an extra internal call to `mktime` is affected by timezone settings.
+    dt1 = withenv("TZ" => "UTC") do
+        Libc.strptime("%Y-%m-%dT%H:%M:%SZ", "2018-11-16T10:26:14Z")
+    end
     dt2 = Libc.TmStruct(14, 30, 5, 10, 1, 99, 3, 40, 0)
 
     time = Time(dt1)