Based on minitest RubyGem.

Example of before this patch:

    - Assertion[1] Failed: Expected 1 to be 2
       Expected: 2
         Actual: 1
    - Assertion[2] Failed: Expected [1, 3] to include 2
       Collection: [1, 3]
           Object: 2

Example of after this patch:

    - Assertion[1]
       Expected: 2
         Actual: 1
    - Assertion[2]
       Expected [1, 3] to include 2.

Merge pull request #4333 from shuujii/use-FrozenError-instead-of-RuntimeError-in-frozen-obj-mod-test

Use `FrozenError` instead of `RuntimeError` in frozen object modification test

Use Rake instead of MiniRake on AppVeyor

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)

Fix class/instance variable name validation

- `@@?` etc are invalid class variable name.
- `@1` etc are invalid instance variable name.

Refine `appveyor.yml`

- Add Visual Studio 2017.
- Enable `shallow_clone` for saving build time.
- Cache extracted WinFlexBison.

Use `FrozenError` instead of `RuntimeError` in `String#rstrip!`

Fix constant name validation

`X!` etc are invalid constant name.

Do not raise an exception when bintest fail

- An exception do not raise when mrbtest fail.
- There are no useful informations in exception message and backtrace.

Fix typo in `AUTHORS` [ci skip]

Reduce `String` creation in `check_(cv|const)_name_sym`

Fix missing assertions in `mruby-string-ext` test

Update `AUTHORS` [ci skip]

Rename `MITL` to `LICENSE` in `.yardopts`; ref 67728c15 [ci skip]

Add bintest header

Add a missing file for #4314

Allow `enable_bintest` without `enable_test` in build config

Set `GEMNAME` on bintest

Refer also CXX and CC env vars as linker command in gcc and clang toolchain

ref #4292

Count skip tests