make test returns errors

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Hi @mattn, I using your library for my project.
When I try to run `make test`, task is falling on `mruby-thread`.
So I try to test only your lib with crean mruby and build config from `test_build.rb` file, and I again get error.
```bash

Build summary:

================================================
Config Name: host
Output Directory: build/host
Binaries: mrbc, mrbtest
Included Gems:
mruby-sprintf - standard Kernel#sprintf method
mruby-print - standard print/puts/p
mruby-math - standard Math module
mruby-time - standard Time class
mruby-struct - standard Struct class
mruby-compar-ext - Enumerable module extension
mruby-enum-ext - Enumerable module extension
mruby-fiber - Fiber class
mruby-enumerator - Enumerator class
mruby-string-ext - String class extension
mruby-numeric-ext - Numeric class extension
mruby-array-ext - Array class extension
mruby-hash-ext - Hash class extension
mruby-range-ext - Range class extension
mruby-proc-ext - Proc class extension
mruby-symbol-ext - Symbol class extension
mruby-random - Random class
mruby-object-ext - Object class extension
mruby-objectspace - ObjectSpace class
mruby-enum-lazy - Enumerator::Lazy class
mruby-toplevel-ext - toplevel object (main) methods extension
mruby-compiler - mruby compiler library
mruby-bin-mirb - mirb command
- Binaries: mirb
mruby-error - extensional error handling
mruby-bin-mruby - mruby command
- Binaries: mruby
mruby-bin-strip - irep dump debug section remover command
- Binaries: mruby-strip
mruby-kernel-ext - Kernel module extension
mruby-class-ext - class/module extension
mruby-thread
mruby-bin-mrbc - mruby compiler executable
mruby-test - mruby test
================================================

>>> Test host <<<
mrbtest - Embeddable Ruby Test

..............................................................................................?...................................................................................................................................................................................................................................................................?....................X.rake aborted!
Command Failed: ["build/host/bin/mrbtest"]
rakefile:123:in `block (2 levels) in '
make: *** [test] Error 1
mruby_build|master ⇒
```

and here is build config
```ruby
MRuby::Build.new do |conf|
MRBGEMS_ROOT = "/usr/local/mrblib"
toolchain :clang
conf.cc.defines += %w(ENABLE_READLINE)
conf.cc.include_paths << %w(/usr/local/include)
conf.linker.library_paths << %w(/usr/local/lib)
conf.linker.libraries << ['pthread']
conf.gembox 'default'
conf.gem File.dirname(__FILE__)
conf.enable_test
end
```
But in my build config I get the same errors.

Reproduce `make test` with the `test_build.rb` configuration and inspect the failure from `mruby-thread`, especially `build/host/bin/mrbtest`. Compare the failing run with the standalone library build, then follow the test command in `rakefile:123`; done means the reported test command completes successfully or the specific failing test and cause are identified.

Written by the indexing model from the issue text.