`IRB_USE_AUTOCOMPLETE=false`
(https://github.com/ruby/irb/pull/469)

* Allow using IRB_USE_AUTOCOMPLETE=false to disable autocompletion

Currently, the only 2 ways to disable autocompletion are:

1. Create `.irbrc` and set `IRB.conf[:USE_AUTOCOMPLETE] = false`
2. Add the `--noautocomplete` flag when using the `irb` executable

Both of them are less convenient than setting a env var and are
lesser known to devs.

And given the number of problems the autocompletion has (see #445), I
think we should allow disabling it with a simple `IRB_USE_AUTOCOMPLETE=false`.

* Mention some env var configs in the README

conveniently
(https://github.com/ruby/irb/pull/429)

* Create a base TestIRB::TestCase class

* Save/restore encodings for tests that initializes InputMethod classes

Because `RelineInputMethod#initializes` calls `set_encoding`, which
changes stdio/out/err and Encoding's default encoding values, we need to
make sure any test that directly or indirectly (e.g. through Context)
initializes `RelineInputMethod` restores encodings.

`ReadlineInputMethod` also changes encodings but currently no tests
cover it.

* Remove unnecessary TestHelper module

Since we now have a base TestCase, without_rdoc can just live there.

https://github.com/ruby/irb/commit/c2874ec121

(https://github.com/ruby/irb/pull/357)


https://github.com/ruby/irb/commit/298b134792

https://no-color.org has been updated (jcs/no_color#83):

> Command-line software which adds ANSI color to its output by default
should check for a `NO_COLOR` environment variable that, when present
and **not an empty string** (regardless of its value), prevents the
addition of ANSI color.

https://github.com/ruby/irb/commit/46e0f7e370

Co-authored-by: Stan Lo <stan001212@gmail.com>

Also add --script option to turn the option back on.

Previously there wasn't a way to get an interactive IRB session
and access arguments provided on the command line.

Additionally, handle `-` as script as stdin. In Unix-like tools, `-`
means to take standard input instead of a file.  This doesn't
result in exactly the same output for:

```
echo 'p ARGV' > args.rb; irb args.rb a b c
```

and

```
echo 'p ARGV' | irb - a b c
```

Due to how irb handles whether stdin is a tty.

However, this change allows use of `-` as a argument, instead of
giving an unrecognized switch error. This required some small
changes to context.rb (to handle `-` as standard input) and
input-method.rb (to have FileInputMethod accept IO arguments in
addition to strings).

Implements [Feature #15371]

https://github.com/ruby/irb/commit/4192683ba2

The test randomly gets stuck on Solaris:

http://rubyci.s3.amazonaws.com/solaris11-sunc/ruby-master/log/20211105T060014Z.fail.html.gz
```
  1) Error:
TestIRB::TestInit#test_recovery_sigint:
Timeout::Error: execution of assert_in_out_err expired timeout (100.0 sec)
pid 3195 killed by SIGTERM (signal 15)
| Switch to inspect mode.
| exit
|
```

I investigated the issue but I couldn't figure it out.
This random failure is noisy, which makes it difficult to track the CI.
So I skip the test on Soalris. Contribution is welcome.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

https://github.com/ruby/irb/commit/2dfdc031ec

When `NO_COLOR` is set to any non-nil value, output is not colorized.

See https://no-color.org/

https://github.com/ruby/irb/commit/401d0916fe

Not to be affected by existing rc files in all tests.

https://github.com/ruby/irb/commit/bf434892b4