tests/provisioners: Add TestProvsionerPreCheck function

This function can be used to check if a Provisioner has been marked for testing within the ACC_TEST_PROVISIONERS environment variable.

While testing I found that the shell acceptance test were also running when trying to run powershell tests.

Before change
```
⇶  ACC_TEST_BUILDERS=amazon-ebs ACC_TEST_PROVISIONERS=powershell go test -v ./provisioner/shell/... -timeout=1h

=== RUN
TestShellProvisioner/testing_amazon-ebs_builder_against_shell_provisioner
2020/04/06 15:18:12 ui: amazon-ebs: output will be in this color.
2020/04/06 15:18:12 ui:
2020/04/06 15:18:12 Build debug mode: false
2020/04/06 15:18:12 Force build: false
2020/04/06 15:18:12 On error:
2020/04/06 15:18:12 Preparing build: amazon-ebs
2020/04/06 15:18:12 Waiting on builds to complete...
2020/04/06 15:18:12 Starting build run: amazon-ebs
2020/04/06 15:18:12 Running builder: amazon-ebs
```

After changes
```
⇶  ACC_TEST_BUILDERS=amazon-ebs ACC_TEST_PROVISIONERS=powershell go test -v ./provisioner/shell/... -timeout=1h
--- SKIP: TestShellProvisioner (0.00s)
    provisioners.go:88: Provisioner "shell" not defined in ACC_TEST_PROVISIONERS
```

helper/tests/acc/provisioners.go

@@ -3,13 +3,14 @@ package acc
 import (
 	"bytes"
 	"fmt"
-	"github.com/hashicorp/packer/helper/tests"
 	"log"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 
+	testshelper "github.com/hashicorp/packer/helper/tests"
+
 	amazonEBS "github.com/hashicorp/packer/builder/amazon/ebs/acceptance"
 	virtualboxISO "github.com/hashicorp/packer/builder/virtualbox/iso/acceptance"
 	"github.com/hashicorp/packer/command"
@@ -68,6 +69,26 @@ func TestProvisionersAgainstBuilders(provisionerAcc ProvisionerAcceptance, t *te
 	}
 }
 
+// TestProvisionersPreCheck checks if the Provisioner with name is set in ACC_TEST_PROVISIONERS environment variable
+func TestProvisionersPreCheck(name string, t *testing.T) {
+	p := os.Getenv("ACC_TEST_PROVISIONERS")
+
+	if p == "all" {
+		return
+	}
+
+	provisioners := strings.Split(p, ",")
+	for _, provisioner := range provisioners {
+		if provisioner == name {
+			return
+		}
+	}
+
+	msg := fmt.Sprintf("Provisioner %q not defined in ACC_TEST_PROVISIONERS", name)
+	t.Skip(msg)
+
+}
+
 func checkBuilders(t *testing.T) []string {
 	b := os.Getenv("ACC_TEST_BUILDERS")
 	// validate if we want to run provisioners acc tests

provisioner/powershell/provisioner_acc_test.go

@@ -6,7 +6,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"strings"
 	"testing"
 
 	"github.com/hashicorp/packer/provisioner/powershell"
@@ -17,21 +16,17 @@ import (
 	"github.com/hashicorp/packer/packer"
 )
 
+const TestProvisionerName = "powershell"
+
 func TestPowershellProvisioner_Inline(t *testing.T) {
-	p := os.Getenv("ACC_TEST_PROVISIONERS")
-	if p != "all" && !strings.Contains(p, "powershell") {
-		t.Skip()
-	}
+	acc.TestProvisionersPreCheck(TestProvisionerName, t)
 
 	testProvisioner := PowershellProvisionerAccTest{"powershell-inline-provisioner.txt"}
 	acc.TestProvisionersAgainstBuilders(&testProvisioner, t)
 }
 
 func TestPowershellProvisioner_Script(t *testing.T) {
-	p := os.Getenv("ACC_TEST_PROVISIONERS")
-	if p != "all" && !strings.Contains(p, "powershell") {
-		t.Skip()
-	}
+	acc.TestProvisionersPreCheck(TestProvisionerName, t)
 
 	testProvisioner := PowershellProvisionerAccTest{"powershell-script-provisioner.txt"}
 	acc.TestProvisionersAgainstBuilders(&testProvisioner, t)
@@ -42,7 +37,7 @@ type PowershellProvisionerAccTest struct {
 }
 
 func (s *PowershellProvisionerAccTest) GetName() string {
-	return "powershell"
+	return TestProvisionerName
 }
 
 func (s *PowershellProvisionerAccTest) GetConfig() (string, error) {
@@ -59,7 +54,7 @@ func (s *PowershellProvisionerAccTest) GetConfig() (string, error) {
 
 func (s *PowershellProvisionerAccTest) GetProvisionerStore() packer.MapOfProvisioner {
 	return packer.MapOfProvisioner{
-		"powershell": func() (packer.Provisioner, error) { return &powershell.Provisioner{}, nil },
+		TestProvisionerName: func() (packer.Provisioner, error) { return &powershell.Provisioner{}, nil },
 	}
 }
 

provisioner/shell/provisioner_acc_test.go

@@ -6,7 +6,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"strings"
 	"testing"
 
 	"github.com/hashicorp/packer/helper/tests/acc"
@@ -21,10 +20,7 @@ import (
 )
 
 func TestShellProvisioner(t *testing.T) {
-	p := os.Getenv("ACC_TEST_PROVISIONERS")
-	if p != "all" && !strings.Contains(p, "shell") {
-		t.Skip()
-	}
+	acc.TestProvisionersPreCheck("shell", t)
 	acc.TestProvisionersAgainstBuilders(new(ShellProvisionerAccTest), t)
 }