Skip test 1623 if '~' not defined (#6577)

* skip test 1623 if no '~' (fix #6576)

* document change

Author: mfansler

inst/tests/tests.Rraw

@@ -8912,8 +8912,8 @@ ans2[ , (date_cols) := lapply(.SD, as.IDate), .SDcols = date_cols]
 test(1622.1, ans1, ans2)
 test(1622.2, ans1, fread(testDir("issue_1573_fill.txt"), fill=TRUE, sep=" ", na.strings=""))
 
-# fix for #989
-test(1623, fread("~"), error="File '~' is a directory")
+# fix for #989; possibly skip for #6576
+if (dir.exists("~")) test(1623, fread("~"), error="File '~' is a directory")
 
 # testing print.rownames option, #1097 (part of #1523)
 options(datatable.print.rownames = FALSE)