test: copy files in test_expect_equal_file instead of moving them

test: copy files in test_expect_equal_file instead of moving them

Before the change, test_expect_equal_file moved files it compared
in case of failure.  The patch changes it to copy the files
instead.  This allows testing non-temporary files which are
stored in git.

Note: the change should not result in new temporary files left
after the tests.  Test_expect_equal_file used to move files only
on failure, so callers had to cleanup them anyway.