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/testing.txt

@@ -11,6 +11,8 @@
             c
         failed
         after failed
+    --- FAIL: TestBar/Bar4 (0.00s)
+        fatal
     log Bar end
 --- FAIL: TestAllLowercase (0.00s)
     --- FAIL: TestAllLowercase/BETA (0.00s)