internal/core/adt: revert the top-level parallelism of EvalV{1,2} tests

Otherwise using CUE_UPDATE=1 would cause races when updating the outputs
in txtar files, as TestEvalV3 relies on the results of TestEvalV2.

`go test` slows down a bit due to this, from ~2s to ~2.2s on my machine,
but it's not too bad given that all the sub-tests still run in parallel
amongst themselves within each top-level test.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ie530257985d15eeadb2f684136ce6e6610bb7aaf
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1219274
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mvdan

internal/core/adt/eval_test.go

@@ -44,8 +44,12 @@ var (
 )
 
 // TestEvalV2 tests the old implementation of the evaluator.
+// Note that [TestEvalV3] with CUE_UPDATE=1 assumes it runs after this test
+// for the sake of comparing results between the two evaluator versions.
+// As such, these two tests are not parallel at the top level.
+//
+// Note that this also means that CUE_UPDATE=1 is broken under `go test -shuffle`.
 func TestEvalV2(t *testing.T) {
-	t.Parallel()
 	test := cuetxtar.TxTarTest{
 		Root: "../../../cue/testdata",
 		Name: "eval",
@@ -67,7 +71,6 @@ func TestEvalV2(t *testing.T) {
 }
 
 func TestEvalV3(t *testing.T) {
-	t.Parallel()
 	adt.DebugDeps = true // check unmatched dependencies.
 
 	test := cuetxtar.TxTarTest{