Remove unneeded `=>` in test skip/error messages

Remove unneeded `const_defined?(:Time)` in `mruby-io` test

`mruby-time` is included in test dependencies.

Remove explicit set of `DISABLE_GEMS`

Refine mrbgem name in assertion failure/skip message for core test

Refactor `src/backtrace.c`

- Move calling `mrb_debug_get_filename()` to after `lineno` check.
- Remove unneeded array check in `print_backtrace()`.
- Add a few `const` qualifier.

Move `Object#dig` to `Struct#dig`

This method seems to be mistakenly put into `Object` instead of `Struct`
since it's in `struct.rb` and daf83946 says:

  add #dig to Array,Hash and Struct

`DISABLE_GEMS` is automatically set (or unset); ref #790

Integrate `init_mrbtest.c` to `driver.c` in `mruby-test` mrbgem

- `mrbgemtest_init()` is needed if `DISABLE_GEMS` is enabled because core
  tests are run as part of `mruby-test` mrbgem (moreover, `DISABLE_GEMS` is
  disabled when `enable_test` is used in build config).
- For the same reason `mrb_open_core()` etc for core tests is unneeded.

Fix typo in `lib/mruby/build/command.rb`

Refactor exception handling in `assert`

`assert_true`/`assert_false` should pass when actual is only `true`/`false`

For the following reasons:

- Previous behavior is confusable because it's different from test/unit rubygem's `assert_true`
- Tests may pass unintentionally in an inappropriate way; ref #4285 #4287

mrdb: add "info locals" command

Use more appropriate assertion methods

Merge pull request #4284 from shuujii/use-assert_same-instead-of-assert_equal-in-enum-reverse_each-test

Use `assert_same` instead of `assert_equal` in `Enumerable#reverse_each` test

Fix wrong assertion method in some tests

Reorder members in `struct backtrace_location`

Remove unneeded `memset()` in `src/backtrace.c`

`sizeof(struct backtrace_location)` is 24 bytes -> 16 bytes in LP64
data model etc.

Fix destroyed "inline packed symbols" on 32 bit mode with `MRB_WORD_BOXING`

Refine dependencies for `mruby-config`

Fix typos