packer_test: add FileExists checker

Some tests will create files and directories as part of the execution
path for Packer, and we need a way to check this, so this commit adds a
new file gadget to do those checks after a command executes.

packer_test/common/check/file_gadgets.go

@@ -0,0 +1,35 @@
+package check
+
+import (
+	"fmt"
+	"os"
+)
+
+type fileExists struct {
+	filepath string
+	isDir    bool
+}
+
+func (fe fileExists) Check(_, _ string, _ error) error {
+	st, err := os.Stat(fe.filepath)
+	if err != nil {
+		return fmt.Errorf("failed to stat %q: %s", fe.filepath, err)
+	}
+
+	if st.IsDir() && !fe.isDir {
+		return fmt.Errorf("file %q is a directory, wasn't supposed to be", fe.filepath)
+	}
+
+	if !st.IsDir() && fe.isDir {
+		return fmt.Errorf("file %q is not a directory, was supposed to be", fe.filepath)
+	}
+
+	return nil
+}
+
+func FileExists(filePath string, isDir bool) Checker {
+	return fileExists{
+		filepath: filePath,
+		isDir:    isDir,
+	}
+}