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-wasm.txt

@@ -0,0 +1,21 @@
+--- FAIL: TestBar (0.00s)
+    log Bar
+    log g
+        h
+        i
+        
+    --- FAIL: TestBar/Bar2 (0.00s)
+        log Bar2
+            a
+            b
+            c
+        failed
+        after failed
+    log Bar end
+--- FAIL: TestAllLowercase (0.00s)
+    --- FAIL: TestAllLowercase/BETA (0.00s)
+        expected lowercase name, got BETA
+    --- FAIL: TestAllLowercase/BELTA (0.00s)
+        expected lowercase name, got BELTA
+FAIL
+exitcode: 1