patch 8.1.1567: localtime_r() does not respond to $TZ changes

Problem:    Localtime_r() does not respond to $TZ changes.
Solution:   If $TZ changes then call tzset(). (Tom Ryder)

src/testdir/test_functions.vim

@@ -187,6 +187,30 @@ func Test_strftime()
 
   call assert_fails('call strftime([])', 'E730:')
   call assert_fails('call strftime("%Y", [])', 'E745:')
+
+  " Check that the time changes after we change the timezone
+  " Save previous timezone value, if any
+  if exists('$TZ')
+    let tz = $TZ
+  endif
+
+  " Force EST and then UTC, save the current hour (24-hour clock) for each
+  let $TZ = 'EST' | let est = strftime('%H')
+  let $TZ = 'UTC' | let utc = strftime('%H')
+
+  " Those hours should be two bytes long, and should not be the same; if they
+  " are, a tzset(3) call may have failed somewhere
+  call assert_equal(strlen(est), 2)
+  call assert_equal(strlen(utc), 2)
+  call assert_notequal(est, utc)
+
+  " If we cached a timezone value, put it back, otherwise clear it
+  if exists('tz')
+    let $TZ = tz
+  else
+    unlet $TZ
+  endif
+
 endfunc
 
 func Test_resolve_unix()