Switch from MiniTest to Minitest

The Minitest alias was added 14 years ago in minitest 2.12.0, they
got swapped in 2013 in 5.0.0, and the MiniTest alias inow seems to
no longer exist even if I could not find when it went away.

ruby/t/tc_canvas.rb

@@ -1,6 +1,6 @@
 require 'caca'
 
-class TC_Canvas < MiniTest::Test
+class TC_Canvas < Minitest::Test
     def setup
         @c = Caca::Canvas.new(3, 3)
     end

ruby/t/tc_display.rb

@@ -1,6 +1,6 @@
 require 'caca'
 
-class TC_Canvas < MiniTest::Test
+class TC_Canvas < Minitest::Test
     def test_create
         d = Caca::Display.new()
         refute_nil(d, 'Display creation failed')

ruby/t/tc_dither.rb

@@ -1,6 +1,6 @@
 require 'caca'
 
-class TC_Canvas < MiniTest::Test
+class TC_Canvas < Minitest::Test
     def test_create
         d = Caca::Dither.new(8, 32, 32, 32, 0, 0, 0, 0)
     end

ruby/t/tc_font.rb

@@ -1,6 +1,6 @@
 require 'caca'
 
-class TC_Canvas < MiniTest::Test
+class TC_Canvas < Minitest::Test
     def test_list
         refute_nil(Caca::Font.list)
     end

ruby/t/tc_frame.rb

@@ -1,6 +1,6 @@
 require 'caca'
 
-class TC_Frame < MiniTest::Test
+class TC_Frame < Minitest::Test
     def setup
         @c = Caca::Canvas.new(3, 3)
     end