Remove use of `system`. Add test coverage.

plugins/hosts/void/cap/nfs.rb

@@ -15,7 +15,8 @@ module VagrantPlugins
         end
 
         def self.nfs_installed(env)
-          Kernel.system("/usr/bin/xbps-query nfs-utils", [:out, :err] => "/dev/null")
+          result = Vagrant::Util::Subprocess.execute("/usr/bin/xbps-query nfs-utils")
+          result.exit_code == 0
         end
       end
     end

test/unit/plugins/hosts/void/cap/nfs_test.rb

@@ -0,0 +1,62 @@
+require_relative "../../../../base"
+require_relative "../../../../../../plugins/hosts/void/cap/nfs"
+require_relative "../../../../../../lib/vagrant/util"
+
+describe VagrantPlugins::HostVoid::Cap::NFS do
+
+  include_context "unit"
+
+  let(:caps) do
+    VagrantPlugins::HostVoid::Plugin
+      .components
+      .host_capabilities[:void]
+  end
+
+  let(:env) { double("env") }
+
+  context ".nfs_check_command" do
+    it "should provide nfs_check_command capability" do
+      expect(caps.get(:nfs_check_command)).to eq(described_class)
+    end
+
+    it "should return command to execute" do
+      expect(caps.get(:nfs_check_command).nfs_check_command(env)).to be_a(String)
+    end
+  end
+
+  context ".nfs_start_command" do
+    it "should provide nfs_start_command capability" do
+      expect(caps.get(:nfs_start_command)).to eq(described_class)
+    end
+
+    it "should return command to execute" do
+      expect(caps.get(:nfs_start_command).nfs_start_command(env)).to be_a(String)
+    end
+  end
+
+  context ".nfs_installed" do
+    let(:exit_code) { 0 }
+    let(:result) { Vagrant::Util::Subprocess::Result.new(exit_code, "", "") }
+
+    before { allow(Vagrant::Util::Subprocess).to receive(:execute).
+        with(/xbps-query nfs-utils/).and_return(result) }
+
+    it "should provide nfs_installed capability" do
+      expect(caps.get(:nfs_installed)).to eq(described_class)
+    end
+
+    context "when installed" do
+      it "should return true" do
+        expect(caps.get(:nfs_installed).nfs_installed(env)).to be_truthy
+      end
+    end
+
+    context "when not installed" do
+      let(:exit_code) { 1 }
+
+      it "should return false" do
+        expect(caps.get(:nfs_installed).nfs_installed(env)).to be_falsey
+      end
+    end
+  end
+end