How can I allow one package access to another package's unexported data only when testing?

In The Go Programming Language, section 11.2.4, there is an example of of an external test accessing fmt.isSpace() via a declaration of IsSpace in fmt's export_test.go file. That seemed like the perfect solution, so that's what I did:

a/a.go:

package a

var x int

func Set(v int) {
    x = v
}

a/a_test.go:

package a
import "testing"

func TestSet(t *testing.T) {
    Set(42)
    if x != 42 {
        t.Errorf("x == %d (expected 42)", x)
    }
}

func Get() int {
    return x
}

(Running go test in a/ works fine.)

b/b.go:

package b
import "path/to/a"

func CallSet() {
    a.Set(105)
}

b/b_test.go

package b
import (
    "testing"
    "path/to/a"
)

func TestCallSet(t *testing.T) {
    CallSet()
    if r := a.Get(); r != 105 {
        t.Errorf("Get() == %d (expected 105)", r)
    }
}

Unfortunately, when I run go test in b/, I get:

./b_test.go:11: undefined: a.Get

Trying to run both sets of test at the same time with go test ./... did not help.

After some considerable poking around I discover that "The *_test.go files are compiled into the package only when running go test for that package" (emphasis mine). (So, in other words I could access a.Get from an a_test external test package in a/, but not from any package outside of a/.)

Is there some other way I can allow tests from one package to access otherwise-internal data from another package, for integration testing purposes?