Add shtesting of timegm and timelocal

tests/shtest

@@ -561,4 +561,19 @@ if ! $VALGRIND $Q $JQ -n -f "$JQTESTDIR/yes-main-program.jq" > $d/out 2>&1; then
   exit 1
 fi
 
+# #2863
+if ! TZ=CST+6CDT $VALGRIND $Q $JQ -cne '
+    1693243637
+  | localtime
+  | (.[9]  |= if .==null then -18000 else . end)
+  | (.[10] |= if .==null then "CDT" else . end)
+  | [.,mktime,timegm,timelocal,timegm - timelocal]
+  | debug
+  | (.[1] == .[2])
+    and
+    (. == [[2023,7,28,12,27,17,1,239,true,-18000,"CDT"],1693225637,1693225637,1693243637,-18000])'; then
+  echo "Time functions not working correctly"
+  exit 1
+fi
+
 exit 0