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

main_test.go

@@ -333,9 +333,6 @@ func runTestWithConfig(name string, t *testing.T, options compileopts.Options, c
 		}
 	}()
 	err = cmd.Wait()
-	if _, ok := err.(*exec.ExitError); ok && options.Target != "" {
-		err = nil // workaround for QEMU
-	}
 	close(runComplete)
 
 	if ranTooLong {