Workaround a zombie process created by Open3

with MJIT worker enabled

The problem:

```
$ ruby -ropen3 --jit -e 'Open3.capture2e("cmake") rescue nil;binding.irb'
irb(main)[01:0]> Process.waitall
=> [[10656, #<Process::Status: pid 10656 exit 127>]]

$ ruby -ropen3 -e 'Open3.capture2e("cmake") rescue nil;binding.irb'
irb(main)[01:0]> Process.waitall
=> []
```

Not sure why it's happening yet, but first I'd like to prevent trunk-mjit-wait
from failing like http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2929075.

test/rubygems/test_gem_ext_cmake_builder.rb

@@ -14,6 +14,14 @@ def setup
       _, status = Open3.capture2e('cmake')
       skip 'cmake not present' unless status.success?
     rescue Errno::ENOENT
+      # Open3.capture2e with ENOENT with JIT enabled leaves a zombie process.
+      # TODO: avoid making the zombie on ENOENT with JIT
+      if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled?
+        begin
+          Process.waitall
+        rescue Errno::ECHILD
+        end
+      end
       skip 'cmake not present'
     end