benchmarks/mst-bench: Update to 0.2.1-18

Increase array test sizes and reps to wash out sampling error on modern
hardware, where run times were becoming too short and inconsistent.
A few other minor enhancements

benchmarks/mst-bench/Makefile

 PORTNAME=	MST-Bench
-DISTVERSION=	0.2.1-3
-DISTVERSIONSUFFIX=	-g0175cb2
+DISTVERSION=	0.2.1-18
+DISTVERSIONSUFFIX=	-gd343ef8
 CATEGORIES=	benchmarks
 
 MAINTAINER=	jwb@FreeBSD.org
-COMMENT=	Maximum sustained throughput benchmark
+COMMENT=	Maximum sustainable throughput benchmark
 
 LICENSE=	BSD2CLAUSE
 LICENSE_FILE=	${WRKSRC}/LICENSE

benchmarks/mst-bench/distinfo

-TIMESTAMP = 1623949342
-SHA256 (outpaddling-MST-Bench-0.2.1-3-g0175cb2_GH0.tar.gz) = 454d48039f25e668fcb1dc7946808761fe43333e7dcad8a468c26ec066b90c4f
-SIZE (outpaddling-MST-Bench-0.2.1-3-g0175cb2_GH0.tar.gz) = 10964
+TIMESTAMP = 1644417886
+SHA256 (outpaddling-MST-Bench-0.2.1-18-gd343ef8_GH0.tar.gz) = 6e14ab32597839465144cbfcdf87acfc8298b8e77490c0623fdbb12214c6434a
+SIZE (outpaddling-MST-Bench-0.2.1-18-gd343ef8_GH0.tar.gz) = 21771

benchmarks/mst-bench/pkg-descr

-MST-bench is a simple program to measure optimal sustained memory and disk
+MST-bench is a simple program to measure optimal sustainable memory and disk
 performance.
 
 Unlike many benchmarks, it does not attempt to simulate naturally occurring