Fix irb_test prompt assertion failure

[tompng]

After changing IRB's behavior with TERM=dumb in https://github.com/ruby/irb/pull/907, test_irb_command_switches_console_to_irb and test_irb_console_config_activates_irb began failing. This pull request fixes it.

With a workaround https://github.com/ruby/irb/pull/943, test does not fail now.

Description

As described in def assert_raw_line_text comment, assert_line_text is not suitable for testing IRB's prompt.

# assert_line_text ignores the prompt line, so we can't use it to assert the prompt transition
# assert_raw_line_text is a workaround for that
def assert_raw_line_text(expectation)

The test was passing before because Reline prints redundant output when TERM is set to dumb, which was problematic while running IRB in emacs shell.