Add .yaml as an lld test suffix

Over time we've started to add inputs and test cases using the .yaml
extension, which seems to be preferred over the .objtxt extension that
we were using initially. One nice thing about using .yaml is that it
triggers existing editor highlighting and formatting support.

Fix two pdb*.yaml test cases that I added that weren't being run as part
of check-lld.

llvm-svn: 306303