Fix compatibility with power_assert 3.0 on olrder rubies

power_assert 3.0 now use pattern matching: ruby/power_assert#57
This cause it to be half defined on Ruby 3.0 and older.

Author: byroot

lib/test/unit/assertions.rb

@@ -12,6 +12,9 @@
 begin
   require 'power_assert'
 rescue LoadError, SyntaxError
+  if defined?(::PowerAssert)
+    ::Object.send(:remove_const, :PowerAssert)
+  end
 end
 
 module Test

lib/test/unit/util/backtracefilter.rb

@@ -1,6 +1,9 @@
 begin
   require 'power_assert'
 rescue LoadError, SyntaxError
+  if defined?(::PowerAssert)
+    ::Object.send(:remove_const, :PowerAssert)
+  end
 end
 
 module Test