ci: cover a build without MRB_UTF8_STRING - #7138

Problem

Every build in CI takes the full-core gembox: the three in
build_config/ci/gcc-clang.rb and the one in build_config/ci/msvc.rb.
full-core pulls in mruby-encoding, and that gem is what defines
MRB_UTF8_STRING:

# mrbgems/mruby-encoding/mrbgem.rake
spec.build.defines << "MRB_UTF8_STRING"

So CI has never built a mruby whose strings index by byte, and the tests
written as the byte-indexed mirror of the UTF-8 ones have never run.
String#scrub no-op on non-UTF-8 build guards itself with
skip if "あ".length == 1 and skipped on every job in the matrix. The
byte-counting halves of mruby-regexp and mruby-string-ext went the same
way.

Both gems assume that build exists. mruby-string-ext:

UTF-8 coverage comes from full-core builds (mruby-encoding defines
MRB_UTF8_STRING); the mirror runs on gemboxes without it.

and mruby-regexp, on why it sets MRB_UTF8_SCAN:

mruby-encoding is what does, and the default gembox carries this gem
without it.

Nothing in CI builds the default gembox. The Cosmopolitan job comes
closest, taking the stdlib gemboxes with no mruby-encoding, but
build_config/cosmopolitan.rb enables neither enable_test nor
enable_bintest, so rake test:run:serial builds no mrbtest for it and
runs no test there.

Change

A fourth build in build_config/ci/gcc-clang.rb on the default gembox.

It rides the existing matrix rather than taking a job of its own, the
way the MRB_REGEXP_UNICODE_CASE coverage on full-debug does, so it
costs no runner and gets checked on Linux, macOS and mingw, under gcc
and clang, on ARM64 and on the one runner where char is unsigned. It
enables tests only: the binaries the default gembox adds are the ones
the bintest on the build above already covers.

Verification

rake -m test:run:serial MRUBY_CONFIG=ci/gcc-clang from a clean build
directory, on Linux with gcc:

The new build passes as it stands, so this adds coverage without
uncovering a failure.

Cost

The new build comes to 270 compilation units against the 883 the three
existing ones come to together, so it adds about a third to the compile
work of every job in the GCC-CLANG matrix.

Job durations over the last eight master runs of this workflow, as a
reference point rather than a prediction, since runners vary:

A third more compile work on the worst mingw run seen there comes out
near eight minutes, inside the 10 minute timeout, and that scaling is
pessimistic because a job's checkout and setup do not grow with the
build. Everything outside mingw sits under three minutes either way.
timeout-minutes: 10 is left alone, but mingw is where the margin is
thinnest, so raising it is a one line change if you would rather have
the room.

Note on the four warnings

The new build reports 4 "no assertion" warnings, all in
mrbgems/mruby-string-ext/test/string.rb, where a test wraps its whole
body in if UTF8STRING and so runs empty when strings index by byte:

• String#inspect of a binary string escapes every byte
• String#chop! on a binary string removes one byte
• String#rindex on a binary string counts bytes
• a needle that spells no character is found nowhere

They are warnings rather than failures, and CI stays green. The rest of
that file writes the same condition as end if UTF8STRING, which
reports nothing. A separate PR will bring those four into that form.

Summary by CodeRabbit

• Tests
  • Added CI coverage using the default environment without UTF-8 string support.
  • Added validation for byte-indexed behavior.
  • Binary tests remain excluded from this build.