1. Reduce build time

Build time becomes less than half. In MiniRake, a way of using fiber may not be good.

2. Synchronize standard output

No synchronized:

    mrbtest - Embeddable Ruby Test
    ...........................
    Total: 1165
    (snip)
     Time: 1.19 seconds
    bintest - Command Binary Test

    .....................
    Total: 21
    (snip)
     Time: 0.39 seconds
    mrbtest - Embeddable Ruby Test

    ...........................
    Total: 1165
    (snip)
     Skip: 23
     Time: 1.15 seconds
    (snip)

    >>> Test cxx_abi <<<

    >>> Bintest host <<<
    >>> Test host <<<

    >>> Test full-debug <<<

    >>> Bintest cxx_abi <<<

Synchronized:

    >>> Test full-debug <<<
    mrbtest - Embeddable Ruby Test
    ...........................
    Total: 1165
    (snip)
     Time: 1.25 seconds

    >>> Test host <<<
    mrbtest - Embeddable Ruby Test
    ...........................
    Total: 1165
    (snip)
     Time: 1.16 seconds

    >>> Bintest host <<<
    bintest - Command Binary Test
    .....................
    Total: 21
    (snip)
     Time: 0.41 seconds

    >>> Test cxx_abi <<<
    mrbtest - Embeddable Ruby Test
    (snip)