Slight re-organization: make test suite YAML files easier to find and move grpcwebclient into "testing" folder

[jhump]

I had hoped to move the test suite YAML files up to a top-level testsuites folder. But the go:embed stuff doesn't allow that: you cannot use ".." in the path, which means we'd also have to drop the .go file into the top-level folder, which would result in an exported/non-internal Go package, which we don't want.

So instead, this adds a sym-link named testsuites that links to the folder where the YAML files are. This also moves the YAML files into a data sub-directory so when you look at the testsuites top-level or cd into it, you just get the YAML files (and not the accompanying Go file).

This also moves grpcwebclient into a new testing folder, along with the contents of testdata, just so it's not so prominent in the root repo, considering it is only used to test the reference implementations.

Last little thing, while I was in the Makefile: I removed the "ignore" items for the license-header tool. I did this for consistency because I don't see such "ignore" flags in other repos. Also, it didn't have any impact on the generated output so removing it was harmless.