Fix remaining realpath test issue with symlinks

Pursuant to 0be7903859, there still
remained one issue with the test when run from within a symlinked
directory after fish gained support for cding into symlinks.

This change should make the test function OK both when the tests are run
out of a PWD containing a symlink in its hierarchy and when run
otherwise.

tests/realpath.in

@@ -11,7 +11,7 @@ end
 
 # A non-existent file relative to $PWD succeeds.
 set -l real_path (builtin realpath nonexistent-file)
-if test "$real_path" = "$PWD/nonexistent-file"
+if test "$real_path" = (realpath $PWD)"/nonexistent-file"
     echo nonexistent-file in PWD correctly converted
 end