Ineffective assertion tests

[nanobowers]

Some assertion tests do not do what they advertise:

$ rg 'assert_raises\([A-Za-z]+,' 
optimist/parser_test.rb
870:    assert_raises(CommandlineError, /--one.*--two/) { @p.parse %w(--one --two) }
907:    assert_raises(CommandlineError, /--one.*--two/) { @p.parse %w(--one) }
908:    assert_raises(CommandlineError, /--one.*--two/) { @p.parse %w(--two) }
916:    assert_raises(CommandlineError, /arg2/) { @p.parse(%w(--arg1)) }
917:    assert_raises(CommandlineError, /arg1/) { @p.parse(%w(--arg2)) }
1227:    assert_raises(CommandlineError, /unknown argument '--lib'/) do
1246:    assert_raises(CommandlineError, /specified multiple times/) do
1351:    assert_raises(RuntimeError, "good") do
1360:    assert_raises(RuntimeError, "good") do

The documentation for assert_raises says it takes one or more exceptions. Oops.
https://rubydoc.info/gems/minitest/Minitest%2FAssertions:assert_raises

thus, the Regexp or String given as the second argument is not doing anything.
These should be changed to use the assert_raises_errmatch helper function which takes a single exception type and a Regexp.

Per discussion with @Fryguy on a different PR there it would be good to improve some of the tests' message checking since that is rarely happening.

@miq-bot add-label test