runtime, testing: support Goexit, SkipNow, FailNow

Keep a pending Goexit separate from the active panic state so recovered
deferred panics do not cancel the original Goexit unwind. Goexit should
also ignore -panic=trap, since it is not a panic.

Wire testing.FailNow and SkipNow through runtime.Goexit, and run tests
and benchmarks in goroutines. This lets Goexit terminate the user
function without skipping test bookkeeping. Add Goexit/recover coverage
and enable crypto/ecdh in the Linux stdlib test list.

testdata/recover.txt

@@ -33,6 +33,11 @@ indirect recover returned: indirect panic
 
 # runtime.Goexit
 Goexit deferred function, recover is nil: true
+Goexit recovered deferred panic: panic during Goexit
+Goexit nested recovered deferred panic: nested panic during Goexit
+Goexit loop recovered deferred panic: loop panic during Goexit
+Goexit loop defer
+Goexit loop defer
 
 # repanic
 inner, repanicking