Fix failures on non-UTF-8 environment [Bug #18077]

Call `IOSpecs.io_fixture` with the default encoding explicitly.
`IOSpecs.closed_io` calls the method without optional `mode` which
is set to UTF-8 by default, while the default external encoding
depends on the locale environment variables.

spec/ruby/core/io/external_encoding_spec.rb

@@ -96,7 +96,9 @@ describe "IO#external_encoding" do
 
   ruby_version_is '3.1' do
     it "can be retrieved from a closed stream" do
-      IOSpecs.closed_io.external_encoding.should equal(Encoding.default_external)
+      io = IOSpecs.io_fixture("lines.txt", "r")
+      io.close
+      io.external_encoding.should equal(Encoding.default_external)
     end
   end
 

spec/ruby/core/io/internal_encoding_spec.rb

@@ -115,7 +115,9 @@ describe "IO#internal_encoding" do
 
   ruby_version_is '3.1' do
     it "can be retrieved from a closed stream" do
-      IOSpecs.closed_io.internal_encoding.should equal(Encoding.default_internal)
+      io = IOSpecs.io_fixture("lines.txt", "r")
+      io.close
+      io.internal_encoding.should equal(Encoding.default_internal)
     end
   end