Internationalization tests: prefer `test ! -r' over `test ! -f'

Internationalization tests: prefer `test ! -r' over `test ! -f'

We now use `test ! -r' rather than `test ! -f' to check that
a file does not exists (`test ! -e' would be even better, but
unfortunately it is unportable to e.g. Solaris /bin/sh).

* tests/pot-copyright.test: Prefer `test ! -r' over `test ! -f'.
* tests/pot-empty.test: Likewise.
* tests/pot-format-csharp1.test: Likewise.
* tests/pot-format-java1.test: Likewise.
* tests/pot-format-java2.test: Likewise.
* tests/pot-format-qt1.test: Likewise.
* tests/pot-format-tcl1.test: Likewise.
* tests/pot-linguas.test: Likewise.
* tests/pot-msgidbugs.test: Likewise.
* tests/pot-no-linguas.test: Likewise.
* tests/pot-noinst.test: Likewise.
* tests/pot-override1.test: Likewise.
* tests/pot-samedir.test: Likewise.
* tests/pot-topsrcdir.test: Likewise.
* tests/pot-xgettext1.test: Likewise.
* tests/pot-xgettext2.test: Likewise.
* tests/pot-xgettext3.test: Likewise.
* tests/pot-xgettext4.test: Likewise.