`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