post-[cold|warm]-reboot test fails because of plymouth-show-run.service failing

Bug Description

The post-cold-reboot and post-warm-reboot tests sometimes fail because the plymouth-show-run service fails to load.

This is the case in recent runs with the Raspberry Pi 2, for instance (see this submission).

The job fails with the following output:

Found 1 failed units

Failed units:
  UNIT                      LOAD   ACTIVE SUB    DESCRIPTION
● plymouth-show-run.service loaded failed failed Display state on splash

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.
1 loaded units listed.

To Reproduce

Run a test plan containing post-cold-reboot and/or post-warm-reboot test on a device such as the RPi2 which makes use of the plymouth-show-run service.

Environment

UC22
Checkbox version: Snap (4.0.0)

Relevant log output

No response

Additional context