tst: fix file descriptor leaks #4849

Job	Run time
testinstall - - ubuntu-latest	4m 39s
teststandard - - ubuntu-latest	31m 40s
testmanuals - - ubuntu-latest	6m 50s
testexpect testmockpkg testspecial test-compile testlibgap testkernel - - ubuntu-latest	4m 1s
testpackages testinstall-loadall - - ubuntu-latest	19m 57s
makemanuals testmakeinstall - GAPPREFIX=/tmp/gapprefix CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 10s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	28m 36s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 18s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	5m 16s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	23m 5s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	3m 45s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	2m 52s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 29s
testmockpkg testinstall - - windows-2019	15m 45s
Send Slack notification on status change	0s
	2h 43m 23s

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