tests: Prefer 'cat <<\EOF' to 'cat <<EOF' whenever possible.

tests: Prefer 'cat <<\EOF' to 'cat <<EOF' whenever possible.

Rationale: Not interpreting $ and \ in the here documents is a better default
than interpreting them.

* gettext-tools/tests/xgettext-*: Write 'cat <<\EOF' instead of 'cat <<EOF',
when no shell dollar-substitution and backslash-interpretation is needed.