For PGO/coverage tests that don't need to build or run an actual artifact, we
can use `-Zno-profiler-runtime` to run the test even when the profiler runtime
is not available.
This new nightly-only flag can be used to toggle fine-grained flags that
control the details of coverage instrumentation.
Currently the only supported flag value is `branch` (or `no-branch`), which is
a placeholder for upcoming support for branch coverage. Other flag values can
be added in the future, to prototype proposed new behaviour, or to enable
special non-default behaviour.