aispec: Show the context code around shouldBe matchers

[xerial]

It's useful to show the context code when test matchers fail:

plan shouldNotBe null

// [current error message]
failed: null should not be null (xxxTest.scala:20)

// [better error message]
failed: plan shouldNotBe null (xxxTest.scala: 20)
[obtained]
null
[actual]
null

[xerial]

Based on the experiment on #3950, we can't introduce this feature without breaking binary compatibility.