tests.cfg: Display default_install variant in test name

This type of installation is commonly used, remove the '@'
before it.

Also update the comment for it.

Signed-off-by: Tom Zhou <zhouqt@gmail.com>
virttest.aexpect: Use /bin/bash instead of /bin/sh

Some particular Linux distrio, such as debian, uses a
shell program called dash. Unfortunately, the dash
program can't work with "dash -c source foobar" command.

Hence let's use '/bin/bash' explicitly, instead of the
ambiguous '/bin/sh' symbol link.

Signed-off-by: Tom Zhou <zhouqt@gmail.com>