Fix a couple of tests

Use subject instead of invalid_subject because the validation assertions
test the subject double.

This also adds an additional check when validating the `size` attribute
because it is only required for disks of type `:disk`.

plugins/kernel_v2/config/disk.rb

@@ -172,7 +172,7 @@ module VagrantPlugins
           end
         end
 
-        if !@size
+        if !@size && type == :disk
           errors << I18n.t("vagrant.config.disk.invalid_size", name: @name, machine: machine.name)
         end
 

test/unit/plugins/kernel_v2/config/disk_test.rb

@@ -32,8 +32,6 @@ describe VagrantPlugins::Kernel_V2::VagrantConfigDisk do
   before do
     env = double("env")
 
-    subject.name = "foo"
-    subject.size = 100
     allow(provider).to receive(:capability?).with(:validate_disk_ext).and_return(true)
     allow(provider).to receive(:capability).with(:validate_disk_ext, "vdi").and_return(true)
     allow(provider).to receive(:capability?).with(:set_default_disk_ext).and_return(true)
@@ -41,6 +39,11 @@ describe VagrantPlugins::Kernel_V2::VagrantConfigDisk do
   end
 
   describe "with defaults" do
+    before do
+      subject.name = "foo"
+      subject.size = 100
+    end
+
     it "is valid with test defaults" do
       subject.finalize!
       assert_valid
@@ -58,18 +61,24 @@ describe VagrantPlugins::Kernel_V2::VagrantConfigDisk do
   end
 
   describe "with an invalid config" do
-    let(:invalid_subject) { described_class.new(type) }
+    before do
+      subject.name = "bar"
+    end
 
     it "raises an error if size not set" do
-      invalid_subject.name = "bar"
       subject.finalize!
       assert_invalid
     end
 
     context "with an invalid disk extension" do
       before do
+        subject.size = 100
+        subject.disk_ext = "fake"
+
         allow(provider).to receive(:capability?).with(:validate_disk_ext).and_return(true)
         allow(provider).to receive(:capability).with(:validate_disk_ext, "fake").and_return(false)
+        allow(provider).to receive(:capability?).with(:default_disk_exts).and_return(true)
+        allow(provider).to receive(:capability).with(:default_disk_exts).and_return(["vdi", "vmdk"])
       end
 
       it "raises an error" do