update spec for pre-check

spec/lib/msf/core/exploit/powershell_spec.rb

@@ -323,14 +323,12 @@ RSpec.describe Msf::Exploit::Powershell do
     end
 
     context 'when method is unknown' do
-      before do
-        subject.datastore['Powershell::method'] = 'blah'
-      end
       it 'should raise an exception' do
         except = false
         begin
+          subject.datastore['Powershell::method'] = 'blah'
           subject.cmd_psh_payload(payload, arch)
-        rescue RuntimeError
+        rescue Msf::OptionValidateError
           except = true
         end
         expect(except).to be_truthy