tests/leaks.sh: increase Linux (standard malloc) tolerance

There are intermittent false failures on the GitHub CI runners on
the 1.0 branch, which is compiled with the OS's malloc (as opposed
to ast vmalloc). Increase the byte tolerance for the leaks test
from 4 to 6 bytes on Linux when compiling with standard malloc.

src/cmd/ksh93/tests/leaks.sh

@@ -37,7 +37,7 @@ then	N=512			# number of iterations for each test
 elif	[[ -f /proc/$$/stat && $(uname) == Linux ]]
 then	N=2048			# number of iterations for each test
 	unit=bytes
-	tolerance=$((4*N))	# tolerate 4 bytes per iteration to account for malloc artefacts
+	tolerance=$((6*N))	# tolerate 6 bytes per iteration to account for malloc artefacts
 	function getmem
 	{
 		cut -f 23 -d ' ' </proc/$$/stat