ptest-cargo.bbclass: Support of cargo workspaces

ptest-cargo.bbclass: Support of cargo workspaces

For complex project, it is very common to have multiple
sub artifacts and so use workspaces, sometimes it has
even no root artifacts (but several bin or lib) and
virtual manifest is used for that.

Long story short, support this case in ptest-cargo class
to look for all test binaries in the current project
and no more those generated by the root Cargo.toml

Signed-off-by: Frederic Martinsons <frederic.martinsons@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>