TEST-23-UNIT-FILE: skip verifying masked unit

TEST-23-UNIT-FILE: skip verifying masked unit

This fixes the following failure:
TEST-23-UNIT-FILE.sh[2408]: + systemd-analyze --recursive-errors=no --man=no verify /usr/lib/systemd/system/sysinit.target.wants/systemd-hwdb-update.service
systemd-analyze[2737]: sys-kernel-config.mount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: proc-sys-fs-binfmt_misc.automount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: dev-hugepages.mount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: sys-kernel-tracing.mount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: sys-kernel-debug.mount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: sys-fs-fuse-connections.mount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: dev-mqueue.mount: symlinks are not allowed for units of this type, rejecting.
systemd-analyze[2737]: Unit systemd-hwdb-update.service is masked.
TEST-23-UNIT-FILE.sh[166]: + :
TEST-23-UNIT-FILE.sh[166]: + kill -0 2408
TEST-23-UNIT-FILE.sh[166]: + wait 2408
TEST-23-UNIT-FILE.sh[166]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/TEST-23-UNIT-FILE.verify-unit-files.sh failed'
TEST-23-UNIT-FILE.sh[166]: Subtest /usr/lib/systemd/tests/testdata/units/TEST-23-UNIT-FILE.verify-unit-files.sh failed