Improve testing infra

[st0012]

There's a huge difference between how tests are run in this repo and after being synced to Ruby CI. And that causes Ruby CI failure like this not being detected in the original PR #441. I think the rule of thumb is to have every test file self-contained, which is hard to verify with the current rake test.

So in this PR I added test_in_isolation task, which runs each test file in isolation. With this task, we would be able to catch the above failure before the PR is merged.

With this new task, I also caught another undiscovered error:

=========================================================================================================================================================================================================================================================================================================================================================================================================================================
Error: test_raise_exception_with_different_encoding_containing_invalid_byte_sequence(TestIRB::TestRaiseNoBacktraceException):
  NoMethodError: undefined method `mktmpdir' for Dir:Class

        Dir.mktmpdir("test_irb_raise_no_backtrace_exception_#{$$}") do |tmpdir|
           ^^^^^^^^^
  Did you mean?  mkdir
/Users/hung-wulo/src/github.com/ruby/irb/test/irb/test_raise_no_backtrace_exception.rb:28:in `test_raise_exception_with_different_encoding_containing_invalid_byte_sequence'
     25:
     26:     def test_raise_exception_with_different_encoding_containing_invalid_byte_sequence
     27:       backup_home = ENV["HOME"]
  => 28:       Dir.mktmpdir("test_irb_raise_no_backtrace_exception_#{$$}") do |tmpdir|
     29:         ENV["HOME"] = tmpdir
     30:
     31:         bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
=========================================================================================================================================================================================================================================================================================================================================================================================================================================
.
Finished in 0.477298 seconds.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
3 tests, 6 assertions, 0 failures, 1 errors, 0 pendings, 0 omissions, 0 notifications
66.6667% passed
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
6.29 tests/s, 12.57 assertions/s
=====================================================================================================================================================================================
Test 'test/irb/test_raise_no_backtrace_exception.rb' failed when being executed in isolation. Please make sure 'rake test TEST=test/irb/test_raise_no_backtrace_exception.rb' passes.
=====================================================================================================================================================================================

Which is now fixed in the last commit.

[k0kubun]

👍