Mock the PowerShell.executable method in more tests

On *nix systems we might not have powershell installed and then finding
powershell.exe or pwsh.exe fails in some tests. If we instead mock
self.executable to return "powershell", then the tests pass and don't cause any
side effects.

test/unit/vagrant/util/powershell_test.rb

@@ -114,6 +114,8 @@ describe Vagrant::Util::PowerShell do
   describe ".execute" do
     before do
       allow(described_class).to receive(:validate_install!)
+      allow(described_class).to receive(:executable)
+        .and_return("powershell")
       allow(Vagrant::Util::Subprocess).to receive(:execute)
     end
 
@@ -159,6 +161,7 @@ describe Vagrant::Util::PowerShell do
     before do
       allow(described_class).to receive(:validate_install!)
       allow(Vagrant::Util::Subprocess).to receive(:execute).and_return(result)
+      allow(described_class).to receive(:executable).and_return("powershell")
     end
 
     it "should validate installation before use" do
@@ -223,6 +226,7 @@ describe Vagrant::Util::PowerShell do
     before do
       allow(described_class).to receive(:validate_install!)
       allow(Vagrant::Util::Subprocess).to receive(:execute).and_return(result)
+      allow(described_class).to receive(:executable).and_return("powershell")
     end
 
     it "should validate installation before use" do