test-framework-sh: Fix test failure with AIX 7.1 diff.

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.