twister: enhance support for board@revision as platform name

This adds an extra layer to match the revision part where this
needs to be of permitted patterns as described in.
cmake/modules/extensions.cmake. Without this, the matching
may produce undesirable results. For example, if there is
a file named qemu_x86_tiny_1.conf, the testplan will create
a board name qemu_x86@tiny.1 (which is definitely not correct).

Signed-off-by: Daniel Leung <daniel.leung@intel.com>