[jnigen] Bunch of small testsuite improvements (https://github.com/dart-lang/jnigen/issues/257)

* Throw error in test if summarizer is stale
* Fail if jni dylib is stale
* Add spawnIfNotExists. (It sounds like SQL, which means no further evolution is possible)
* Add tags mechanism to jnigen test
* Add `summarizer_test` and `large_test` tags. So that in dev env, can
exclude these with -x tag for fast iteration.
* Adds dart_test.yaml which is required for this configuration by test
package.
* Use random seed for jnigen tests in CI