`schema_validator_tool` should not be `testonly`

Currently the `schema_validator_tool` is built using `envoy_cc_test` which marks it `testonly` and it in turn depends on a bunch of test libs that are likewise `testonly`

this should not be the case given that this tool is published, and means that it cannot be depended upon as a build target by other non-test build targets

i made an inexpert attempt at doing this, but found myself copying out increasingly large chunks of the test framework. I gave up when i needed to start copying out the test thread code