Fix `Testing` section in dev guidelines #5493

Job	Run time
Cross compiler for s390x	27m 37s
Test in OpenBSD	8m 54s
testinstall - - ubuntu-latest	4m 58s
teststandard - - ubuntu-latest	19m 6s
testmanuals - - ubuntu-latest	5m 55s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 47s
testpackages testinstall-loadall - - ubuntu-latest	28m 57s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	8m 52s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	18m 9s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 8s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	3m 20s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	28m 15s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	6m 48s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	3m 53s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 20s
testmockpkg testinstall - - windows-2019	15m 1s
Send Slack notification on status change	0s
	3h 18m 0s

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