test: temporarily adjust the default mount rate limit

(Hopefully) a temporary workaround for systemd#30573 where the starting the
user session when PID 1 is rate limited stalls even after it leaves the
rate limited state:

[   11.658201] H systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=UnitRemoved cookie=4208 reply_cookie=0 signature=so error-name=n/a error-mes>
[   11.658233] H systemd[1]: Event source 0x559babdd8bb0 (mount-monitor-dispatch) left rate limit state.
[  101.562697] H busctl[784]: Failed to get credentials: Transport endpoint is not connected
[  101.563480] H systemd[1]: systemd-journald.service: Got notification message from PID 300 (WATCHDOG=1)
[  101.563725] H testsuite-74.sh[784]: BusAddress=unixexec:path=systemd-run,argv1=-M.host,argv2=-PGq,argv3=--wait,argv4=-pUser%3dtestuser,argv5=-pPAMName%3dlogin,argv6=systemd-stdio-bridge,argv7=-punix:path%3d%24%7bXDG_RUNTIME_DIR%7d/bus
[  101.564136] H systemd[1]: Successfully forked off '(sd-expire)' as PID 787.
[  101.564754] H systemd[1]: Successfully forked off '(sd-expire)' as PID 788.
[  101.564831] H testsuite-74.sh[381]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/testsuite-74.busctl.sh failed'

The issue appeared after ee07fff which does a bunch of mounts/umounts
that get is into the rate limited state, and is frequent enough to be
annoying, so let's temporarily bump the rate limit to alleviate that.

test/TEST-74-AUX-UTILS/test.sh

@@ -8,6 +8,9 @@ NSPAWN_ARGUMENTS="--private-network"
 # shellcheck source=test/test-functions
 . "${TEST_BASE_DIR:?}/test-functions"
 
+# (Hopefully) a temporary workaround for https://github.com/systemd/systemd/issues/30573
+KERNEL_APPEND="${KERNEL_APPEND:-} SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST=100"
+
 test_append_files() {
     local workspace="${1:?}"