spec: skip ruby_exe tests for wasi due to lack of subprocess

Merged: https://github.com/ruby/ruby/pull/5654

spec/mspec/lib/mspec/guards/platform.rb

@@ -41,6 +41,10 @@ class PlatformGuard < SpecGuard
     os?(:windows)
   end
 
+  def self.wasi?
+    os?(:wasi)
+  end
+
   def self.wsl?
     if defined?(@wsl_p)
       @wsl_p

spec/mspec/lib/mspec/helpers/ruby_exe.rb

@@ -112,6 +112,8 @@ unless Object.const_defined?(:RUBY_EXE) and RUBY_EXE
 end
 
 def ruby_exe(code = :not_given, opts = {})
+  skip "WASI doesn't provide subprocess" if PlatformGuard.wasi?
+
   if opts[:dir]
     raise "ruby_exe(..., dir: dir) is no longer supported, use Dir.chdir"
   end