tests/path.sh: fix spurious 'whence -a' test failures

A recent change in Github's Ubuntu test runners exposed a problem
in the way the all_paths test function replicates 'whence -a'
functionality, causing spurious regression test failures.
The problem occurs when e.g. /bin is a symlink to /usr/bin, but
both are in $PATH. The all_paths function treats them as separate
but 'whence -a' detects a duplicate and will not output /usr/bin/*.
I have not succeeded in making all_paths match 'whence -a' exactly.

src/cmd/ksh93/tests/path.sh: function all_paths:
- Using the -ef test expression operator, remove entries from $PATH
  that point to the same directory as another entry in $PATH (e.g.
  when /bin is a symlink to /usr/bin but both are in $PATH).
  Avoiding such dupes works around the problem.

src/cmd/ksh93/tests/path.sh

@@ -29,6 +29,28 @@ Command=${0##*/}
 integer Errors=0
 
 [[ -d $tmp && -w $tmp && $tmp == "$PWD" ]] || { err\_exit "$LINENO" '$tmp not set; run this from shtests. Aborting.'; exit 1; }
+
+# to avoid spurious test failures with 'whence -a' tests, we need
+# to remove any duplicate paths to the same directory from $PATH.
+function rm_path_dups
+{
+	typeset IFS=':' p seen s newpath
+	set -o noglob
+	p=$PATH:	# IFS field splitting discards a final empty field; add one to avoid that
+	for p in $p
+	do	if	[[ -z $p ]]
+		then	# empty $PATH element == current dir
+			p='.'
+		fi
+		for	s in $seen
+		do	[[ $p -ef $s ]] && continue 2
+		done
+		newpath=${newpath:+$newpath:}$p
+		seen=$seen$p:
+	done
+	PATH=$newpath
+}
+rm_path_dups
 PATH_orig=$PATH
 
 # output all paths to a command, skipping duplicates in $PATH