Add a test case that documents the current behaviour.

This illustrates #361.

The resulting time stamp is the default time (1980) because the
date is invalid in the time zone.

regress/mtime-dst.test

@@ -0,0 +1,7 @@
+description change method from stored to deflated, which will change the timestamp because it is at a DST point and not valid in that time zone
+environment-set TZ America/New_York
+# setting it to a European timezone, where there is no DST change at this point, works
+#environment-set TZ Europe/Zurich
+return 0
+arguments test.zzip  set_file_compression 0 deflate 0
+file test.zzip mtime-dstpoint.zip mtime-default.zip