defmt-test: hide unit test output unless it panicked

[japaric]

it would be nice if probe-run could replicate this behavior of the standard test runner

Desired behavior

#[test]
fn passes() {
    trace!("this is OK!");
}

#[test]
fn fails() {
    trace!("this fails");
    panic!()
}

$ cargo t
0.000000 INFO  (1/2) running `passes`...
0.000002 INFO  (2/2) running `fails`...
0.000003 TRACE this fails
0.000004 ERROR explicit panic (..)

note that timestamp 1 is missing from the output. the device does execute the log statement but probe-run hides it

---

the hard part of implementing this is making probe-run aware (a) that is running a defmt-test binary and (b) of where test functions start and end. I think this could be implemented by adding a custom tag to the defmt-test log statement (that says "({}/{}) running .."); probe-run can use those custom tags to determine the boundaries of tests.