main: use emulator exit code instead of parsing test output

This commit changes `tinygo test` to always look at the exit code of the
running test, instead of looking for a "PASS" string at the end of the
output. This is possible now that the binaries running under
qemu-system-arm or qemu-system-riscv32 will signal the correct exit code
when they exit.

As a side effect, this also makes it possible to avoid the "PASS" line
between successful tests. Before:

    $ tinygo test container/heap container/list
    PASS
    ok  	container/heap	0.001s
    PASS
    ok  	container/list	0.001s

After:

    $ tinygo test container/heap container/list
    ok  	container/heap	0.001s
    ok  	container/list	0.001s

The new behavior is more in line with upstream Go:

    go test container/heap container/list
    ok  	container/heap	0.004s
    ok  	container/list	0.004s

src/testing/testing.go

@@ -285,7 +285,9 @@ func (m *M) Run() int {
 	if failures > 0 {
 		fmt.Println("FAIL")
 	} else {
-		fmt.Println("PASS")
+		if flagVerbose {
+			fmt.Println("PASS")
+		}
 	}
 	return failures
 }