gitignore: Add test assets and build path

Currently after generating the assets and build for local testing,
the git starts showing the generated files as untracked. One needs
to be careful and not use `git add .` to avoid adding them accidentally.
Just add these generated files in test/assets and test/build path to
.gitignore to avoid such accidental addition.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>