spec/ruby/core/process/spawn_spec.rb: skip a test on Android

On Android, STDERR seems to be open even its invoker closes it.

http://rubyci.s3.amazonaws.com/android29-x86_64/ruby-master/log/20201001T014315Z.fail.html.gz

```
1)
Process.spawn closes STDERR in the child if :err => :close FAILED
Expected (59840): "out\nrescued\n"
         but got: "out\n"
```

spec/ruby/core/process/spawn_spec.rb

@@ -536,7 +536,7 @@ describe "Process.spawn" do
     File.read(@name).should == "glarkbang"
   end
 
-  platform_is_not :windows do
+  platform_is_not :windows, :android do
     it "closes STDERR in the child if :err => :close" do
       File.open(@name, 'w') do |file|
         -> do