virbuftest: use field names when initalizing test info

virbuftest: use field names when initalizing test info

Allow adding new fields without changing all the macros.

Otherwise the compiler complains that not all have been initialized:
../../tests/virbuftest.c:419:5: error: missing field 'arg' initializer [-Werror,-Wmissing-field-initializers]
    DO_TEST_ESCAPE("<td></td><td></td>",
    ^
../../tests/virbuftest.c:414:56: note: expanded from macro 'DO_TEST_ESCAPE'
        struct testBufAddStrData info = { data, expect }; \

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>