[Tails-l10n] Build failed in Jenkins: check_PO_master #4680

See <https://jenkins.tails.boum.org/job/check_PO_master/4680/changes>

Changes:

[intrigeri] Document how (and why) one can trigger a test suite run without rebuilding images (refs: #16063)

[intrigeri] Fix typo.

[intrigeri] Document all the cases when Jenkins runs the full test suite.

------------------------------------------
Started by an SCM change
[EnvInject] - Loading node environment variables.
Building remotely on isobuilder2 in workspace <https://jenkins.tails.boum.org/job/check_PO_master/ws/>
Wiping out workspace first.
Cloning the remote Git repository
Cloning repository gitolite3@???:tails
> git init <https://jenkins.tails.boum.org/job/check_PO_master/ws/> # timeout=10
Fetching upstream changes from gitolite3@???:tails
> git --version # timeout=10
> git -c core.askpass=true fetch --tags --progress gitolite3@???:tails +refs/heads/*:refs/remotes/origin/*
> git config remote.origin.url gitolite3@???:tails # timeout=10
> git config remote.origin.fetch +refs/heads/*:refs/remotes/origin/* # timeout=10
> git config remote.origin.url gitolite3@???:tails # timeout=10
Fetching upstream changes from gitolite3@???:tails
> git -c core.askpass=true fetch --tags --progress gitolite3@???:tails +refs/heads/*:refs/remotes/origin/*
> git rev-parse origin/master^{commit} # timeout=10
Checking out Revision 494ad6fd1fe7de63144869851791224b72feca75 (origin/master)
> git config core.sparsecheckout # timeout=10
> git checkout -f 494ad6fd1fe7de63144869851791224b72feca75
> git rev-list b9ca228796681087009da45d453c183d5f190af7 # timeout=10

[check_PO_master] $ /bin/sh -xe /tmp/hudson6083511087761401325.sh
+ /var/lib/jenkins/tools/slaves/check_po
ERROR: wiki/src/doc/first_steps/introduction_to_gnome_and_the_tails_desktop.es.po:
    i18nspector is not happy:
        E: inconsistent-trailing-newlines msgid 'System settings'
checked files are not clean.
Build step 'Execute shell' marked build as failure