Fixes cppcheck warnings:

[tests/document-loader.c:44]: (style) Unused variable: document
[tests/document-loader.c:45]: (style) Unused variable: uri

Fixes clang analyzer warnings:

smart-converter.c:108:8: warning: Potential leak of memory pointed to by 'out'
                if (!care_about_error)
                     ^~~~~~~~~~~~~~~~

smart-converter.c:337:7: warning: Potential leak of memory pointed to by 'aux'
        aux2 = do_test (aux, NULL, encs, aux_len, &guessed);
        ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

smart-converter.c:339:12: warning: Potential leak of memory pointed to by 'aux2'
        g_assert (guessed == pluma_encoding_get_from_charset ("UTF-16"));
                  ^~~~~~~

taken from
https://git.gnome.org/browse/gedit/commit/?id=de9e9716037a567bda012edda8d6f92f3cdd0ea2

patch pointed out by @pyhalov
closes https://github.com/mate-desktop/pluma/issues/189

Test was still failing because it checked the document content before closing.
This commit forces document flush before this check, making the test successful.

CRs always appear at the same modulus 2 offset in test strings, thus bug
https://github.com/mate-desktop/pluma/issues/100 is never detected.
Setting substring lengths to 3 causes the test to detect bug #100, thus failing,
as it should yet do.

that plugin has never been built in linux anyway due to logic
in configure.ac

This reverts commit 13392e50c61ce3f3d7d04b59677380f935cf3d3c.