style: prefer Minitest to MiniTest

since Minitest 5.19 doesn't load it by default

(cherry picked from commit a29042b)

.github/ISSUE_TEMPLATE/1-bug-report.md

@@ -32,7 +32,7 @@ Here's an example of how you might structure such a script:
 require 'nokogiri'
 require 'minitest/autorun'
 
-class Test < MiniTest::Spec
+class Test < Minitest::Spec
   describe "Node#css" do
     it "should find a div using chained classes" do
       html = <<~HEREDOC

CONTRIBUTING.md

@@ -124,7 +124,7 @@ Note that `rake test` does not compile the native extension, and this is intenti
 bundle exec rake compile test
 ```
 
-To run a focused test, use MiniTest's `TESTOPTS`:
+To run a focused test, use Minitest's `TESTOPTS`:
 
 ``` sh
 bundle exec rake compile test TESTOPTS="-n/test_last_element_child/"

test/helper.rb

@@ -128,7 +128,7 @@ class TestBenchmark < Minitest::BenchSpec
   end
 
   # rubocop:disable Style/ClassVars
-  class TestCase < MiniTest::Spec
+  class TestCase < Minitest::Spec
     include TestBase
 
     COMPACT_EVERY = 20
@@ -272,7 +272,7 @@ def assert_not_send(send_ary, m = nil)
     def pending(msg)
       begin
         yield
-      rescue MiniTest::Assertion
+      rescue Minitest::Assertion
         skip("pending #{msg} [#{caller(2..2).first}]")
       end
       flunk("pending test unexpectedly passed: #{msg} [#{caller(1..1).first}]")