cue: fix output in ExampleContext

If the output comment block of a testable example is not the last
comment block then nothing will be tested. This commit combines the
comment blocks so that the example actually tests against the output.

Fixes #1267
Closes #1385

Signed-off-by: Amelia Downs <adowns@vmware.com>
Change-Id: I2e1d6f0e6445053988f88ff50249071f40744851
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/528130
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

cue/examplecompile_test.go

@@ -44,7 +44,7 @@ func ExampleContext() {
 	// a:     2
 	// b:     3
 	// "a+b": 5
-
+	//
 	// expressions
 	// a + b: 5
 	// a * b: 6