Use fake images for machine tests

In tests that do not start a machine, we can use "fake" images to speed
up tests.  In the case of darwin and Linux, that can be /dev/null.  The
hypervisors don't care.

In the case of Windows, some research will need to be done to determine
the same approach but this is a start.

Signed-off-by: Brent Baude <bbaude@redhat.com>

pkg/machine/e2e/config_linux_test.go

@@ -1,3 +1,14 @@
 package e2e_test
 
+import "os"
+
 const podmanBinary = "../../../bin/podman-remote"
+
+var (
+	fakeImagePath string = os.DevNull
+)
+
+func (i *initMachine) withFakeImage(_ *machineTestBuilder) *initMachine {
+	i.image = fakeImagePath
+	return i
+}