adapt lxc-attach tests & add test for pty logging

adapt lxc-attach tests & add test for pty logging

The current tests for lxc-attach pty allocation and I/O redirection rely on the
standard file descriptors of the test script to refer to a pty. If they do not
the tests are effectively useless with respect to pty allocation. We need a test
environment with the standard file descriptors refering to a pty as well. One
solution is to run this test under the script command.

This commit also adds a test whether pty logging works. This test is only
executed when all standard file descriptors refer to a pty.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>