test-wrap-argv: print diff instead of the incorrectly wrapped file

test-wrap-argv: print diff instead of the incorrectly wrapped file

Commit c9c03ea stopped creating an intermediate file during syntax-check
to save on execution time. It also switched to outputting the whole
incorrectly wrapped file instead of a diff needed to fix it.

Feed the newly wrapped file to diff via a pipe.

Note that fixing it by running test-wrap-argv.pl --in-place or
the unit test with VIR_TEST_REGENERATE_OUTPUT is easier.