This commit fixes several missing interface-checks, where the code did not properly
verify a Hook before calling it.
It also fixes an error where the tracer was not set in the vmconfig during t8n initialization.
It also adds a testcase which verifies the tracing-output during t8n executio. The
testcase currently only checks the jsonlogger, but can be extended.
This change also removes a spurious 'null' output in the tracing, when
tracing result is null.
