test-framework-sh: Fix test failure with AIX 7.1 diff. * tests/init.sh (compare_): Don't use 'diff -u' if it inserts a space in column 1, like 'diff -c' does. * tests/test-init.sh (test_compare): Don't repeat the test from init.sh whether 'diff -u' is used. Instead, test whether the output contains some '@' character.