This is just the meson part, no functional change.
Use meson -D slow-tests=yes to set the default,
or SYSTEMD_SLOW_TESTS=yes build/test-foobar for just one test.
Setting the default is more useful for installed tests.
install_tests = get_option('install-tests')
tests = []
+conf.set10('SYSTEMD_SLOW_TESTS_DEFAULT', get_option('slow-tests'))
+
#####################################################################
if get_option('efi')
option('tests', type : 'combo', choices : ['true', 'unsafe'],
description : 'enable extra tests with =unsafe')
+option('slow-tests', type : 'boolean', value : 'false',
+ description : 'run the slow tests by default')
option('install-tests', type : 'boolean', value : 'false',
description : 'install test executables')