Helper function that returns a .po file which strings will be marked as customized.
function getCustomPoFile() {
return <<<EOF
msgid ""
msgstr ""
"Project-Id-Version: Drupal 8\\n"
"MIME-Version: 1.0\\n"
"Content-Type: text/plain; charset=UTF-8\\n"
"Content-Transfer-Encoding: 8bit\\n"
"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
msgid "January"
msgstr "janvier"
EOF;
}