Do not change TZ env var in tests

This caused the tests to fail in Prof Brian Ripley's M1 Mac setup, due
to timezone issues. Use a random timezone for the 'tzone' on the xts
object being plotted.

inst/tinytest/test-plot.R

@@ -40,19 +40,13 @@ expect_equivalent(range(x), p$get_ylim(), info = info_msg)
 get_xcoords_respects_object_tzone <-
 function()
 {
-  systz <- Sys.getenv("TZ")
-  on.exit(Sys.setenv(TZ = systz), add = TRUE)
+  # random timezone
+  tz <- sample(OlsonNames(), 1)
+  dttm <- seq(as.POSIXct("2023-01-01 01:23:45"), , 1, 5)
+  x <- xts(c(5, 1, 2, 4, 3), dttm, tzone = tz)
 
-  dates <- seq(as.Date("2023-01-01"), by = 1, length.out = 5)
-  x <- xts(c(5, 1, 2, 4, 3), order.by = dates)
-
-  Sys.setenv(TZ = "UTC")
   print(p <- plot(x))
-  expect_identical("UTC", tzone(p$get_xcoords(x)),
-                   info = "TZ = UTC")
-
-  Sys.setenv(TZ = "US/Central")
-  expect_identical("UTC", tzone(p$get_xcoords(x)),
-                   info = "TZ = US/Central")
+  expect_identical(tz, tzone(p$get_xcoords(x)),
+                   info = paste("TZ =", tz))
 }
 get_xcoords_respects_object_tzone()