Add build_mrbtest after config block is evaluated

This allows us to add `enable_test` anywhere in a build target,
without having to worry about the order in which they are included.

Previously, there was a bug that occured when adding 'mruby-test' gem
to dependencies before additional gems.

Instead of adding the 'mruby-test' gem dependency manually to a test build,
we now only need to call `enable_test` in the target. This also allows us to
call `test_enabled?` downstream when running mruby tests ourselves.

/cc #2924

Added version.h.md and re.h.md

Extract mrbtest to binary gem

We have do this because mruby's test files are found using MRUBY_ROOT, like this:
    mrbs = Dir.glob("#{MRUBY_ROOT}/test/t/*.rb")

Typo in mruby-bin-debugger/mrbgem.rake

C++11 Build Error: C++11 requires a space between literal and identifiers

Updated string.h.md

Refactor version macros, and add a global const variable

C string.h API documentation.

run bintests within subfolders of bintest/

Add String#freeze test

Removed old mrbgems_test.rake which was merged into mrbgem spec

Use #nil? instead of == nil.