du: tests: use dollar-single quotes where appropriate

No need for "foo$(printf "\t")", $'\t' is both more readable and still
functional.

Reported-by:	Jamie Landeg-Jones <jamie@catflap.org>

usr.bin/du/tests/du_test.sh

@@ -46,7 +46,7 @@ H_flag_body()
 {
 	local paths1='testdir/A/B testdir/A testdir/C testdir'
 	local paths2='testdir/C/B testdir/C'
-	local lineprefix="^[0-9]+$(printf "\t")"
+	local lineprefix=$'^[0-9]+\t'
 	local sep="\$\n${lineprefix}"
 
 	atf_check mkdir testdir