tests/leaks.sh: increase iterations on Linux

There are one or two leaks that show up intermittently on the
Github runners for the 1.0 branch (which is compiled as a release,
i.e. no vmalloc). If they're intermittent, they must be false
positives due to malloc artefacts. Let's double the number of
iterations for the /proc/$$/stat method and see what happens.

src/cmd/ksh93/tests/leaks.sh

@@ -36,7 +36,7 @@ then	N=512			# number of iterations for each test
 	}
 # On Linux, we can use /proc to get byte granularity for vsize (field 23).
 elif	[[ -f /proc/$$/stat && $(uname) == Linux ]]
-then	N=2048			# number of iterations for each test
+then	N=4096			# number of iterations for each test
 	unit=bytes
 	tolerance=$((8*N))	# tolerate 8 bytes per iteration to account for malloc artefacts
 	function getmem