test/poll-mshot-update: ensure 'nfiles' doesn't exceed NFILES #1500

Job	Run time
build (x86_64, gcc-x86-64-linux-gnu, g++-x86-64-linux-gnu, x86_64-linux-gnu-gcc, x86_64-linux-gnu...	29s
build (x86_64, gcc-x86-64-linux-gnu, g++-x86-64-linux-gnu, x86_64-linux-gnu-gcc, x86_64-linux-gnu...	24s
build (x86_64, clang, clang, clang, clang++, -Wshorten-64-to-32, -Wmissing-prototypes -Wstrict-pr...	49s
build (i686, gcc-i686-linux-gnu, g++-i686-linux-gnu, i686-linux-gnu-gcc, i686-linux-gnu-g++, 0)	38s
build (aarch64, gcc-aarch64-linux-gnu, g++-aarch64-linux-gnu, aarch64-linux-gnu-gcc, aarch64-linu...	33s
build (arm, gcc-arm-linux-gnueabi, g++-arm-linux-gnueabi, arm-linux-gnueabi-gcc, arm-linux-gnueab...	47s
build (riscv64, gcc-riscv64-linux-gnu, g++-riscv64-linux-gnu, riscv64-linux-gnu-gcc, riscv64-linu...	31s
build (powerpc64, gcc-powerpc64-linux-gnu, g++-powerpc64-linux-gnu, powerpc64-linux-gnu-gcc, powe...	31s
build (powerpc, gcc-powerpc-linux-gnu, g++-powerpc-linux-gnu, powerpc-linux-gnu-gcc, powerpc-linu...	42s
build (alpha, gcc-alpha-linux-gnu, g++-alpha-linux-gnu, alpha-linux-gnu-gcc, alpha-linux-gnu-g++, 0)	1m 28s
build (mips64, gcc-mips64-linux-gnuabi64, g++-mips64-linux-gnuabi64, mips64-linux-gnuabi64-gcc, m...	2m 4s
build (mips, gcc-mips-linux-gnu, g++-mips-linux-gnu, mips-linux-gnu-gcc, mips-linux-gnu-g++, 0)	32s
build (hppa, gcc-hppa-linux-gnu, g++-hppa-linux-gnu, hppa-linux-gnu-gcc, hppa-linux-gnu-g++, 0)	30s
	9m 58s

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