testutil: make testing packages public

This was done with something along the lines of:

```
mv internal/test testutil
pushd testutil/; grep -IRl "package test" | xargs -I '{}' sed -i -e 's|package test|package testutil|g' {}; popd
mv internal/testutil/*.go testutil/ && rm -rf internal/
grep -IRl "github.com\/docker\/docker\/internal\/test" | xargs -I '{}' sed -i -e 's|github.com/docker/docker/internal/test|github.com/docker/docker/test|g' {}
goimports .
```

I also modified the basic plugin path in testutil/fixtures/plugin.

Signed-off-by: Sam Whited <sam@samwhited.com>

testutil/fakecontext/context.go

@@ -0,0 +1,131 @@
+package fakecontext // import "github.com/docker/docker/testutil/fakecontext"
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+
+	"github.com/docker/docker/pkg/archive"
+	"github.com/docker/docker/testutil"
+)
+
+type testingT interface {
+	Fatal(args ...interface{})
+	Fatalf(string, ...interface{})
+}
+
+// New creates a fake build context
+func New(t testingT, dir string, modifiers ...func(*Fake) error) *Fake {
+	if ht, ok := t.(testutil.HelperT); ok {
+		ht.Helper()
+	}
+	fakeContext := &Fake{Dir: dir}
+	if dir == "" {
+		if err := newDir(fakeContext); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	for _, modifier := range modifiers {
+		if err := modifier(fakeContext); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	return fakeContext
+}
+
+func newDir(fake *Fake) error {
+	tmp, err := ioutil.TempDir("", "fake-context")
+	if err != nil {
+		return err
+	}
+	if err := os.Chmod(tmp, 0755); err != nil {
+		return err
+	}
+	fake.Dir = tmp
+	return nil
+}
+
+// WithFile adds the specified file (with content) in the build context
+func WithFile(name, content string) func(*Fake) error {
+	return func(ctx *Fake) error {
+		return ctx.Add(name, content)
+	}
+}
+
+// WithDockerfile adds the specified content as Dockerfile in the build context
+func WithDockerfile(content string) func(*Fake) error {
+	return WithFile("Dockerfile", content)
+}
+
+// WithFiles adds the specified files in the build context, content is a string
+func WithFiles(files map[string]string) func(*Fake) error {
+	return func(fakeContext *Fake) error {
+		for file, content := range files {
+			if err := fakeContext.Add(file, content); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+}
+
+// WithBinaryFiles adds the specified files in the build context, content is binary
+func WithBinaryFiles(files map[string]*bytes.Buffer) func(*Fake) error {
+	return func(fakeContext *Fake) error {
+		for file, content := range files {
+			if err := fakeContext.Add(file, content.String()); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+}
+
+// Fake creates directories that can be used as a build context
+type Fake struct {
+	Dir string
+}
+
+// Add a file at a path, creating directories where necessary
+func (f *Fake) Add(file, content string) error {
+	return f.addFile(file, []byte(content))
+}
+
+func (f *Fake) addFile(file string, content []byte) error {
+	fp := filepath.Join(f.Dir, filepath.FromSlash(file))
+	dirpath := filepath.Dir(fp)
+	if dirpath != "." {
+		if err := os.MkdirAll(dirpath, 0755); err != nil {
+			return err
+		}
+	}
+	return ioutil.WriteFile(fp, content, 0644)
+
+}
+
+// Delete a file at a path
+func (f *Fake) Delete(file string) error {
+	fp := filepath.Join(f.Dir, filepath.FromSlash(file))
+	return os.RemoveAll(fp)
+}
+
+// Close deletes the context
+func (f *Fake) Close() error {
+	return os.RemoveAll(f.Dir)
+}
+
+// AsTarReader returns a ReadCloser with the contents of Dir as a tar archive.
+func (f *Fake) AsTarReader(t testingT) io.ReadCloser {
+	if ht, ok := t.(testutil.HelperT); ok {
+		ht.Helper()
+	}
+	reader, err := archive.TarWithOptions(f.Dir, &archive.TarOptions{})
+	if err != nil {
+		t.Fatalf("Failed to create tar from %s: %s", f.Dir, err)
+	}
+	return reader
+}