testing: Consider using CI version matrix instead of hard-coding latest versions

[kmoe]

Instead of maintaining a list of version numbers in internal/testutil/tfcache.go, use a version matrix in the CircleCI config. The TFEXEC_E2ETEST_VERSIONS env var should then be used for similar behaviour during local development.

Note that if we remove these consts entirely, tests not using the main runTest() helper to run tests against the default list of versions will need to hard-code the relevant versions. However, such tests often make use of latest versions, e.g.:

terraform-exec/tfexec/internal/e2etest/upgrade012_test.go

Lines 14 to 16 in cb3a405

	func TestUpgrade012(t *testing.T) {
	runTestVersions(t, []string{testutil.Latest012}, "pre_011_syntax", func(t *testing.T, tfv *version.Version, tf *tfexec.Terraform) {
	err := tf.Init(context.Background())

Perhaps the consts should instead be initialised according to values in the CircleCI environment.

[kmoe]

I'm currently inclined slightly against this, because it makes it a bit more awkward to make sure e2e tests run during local development are using the same versions as CI.

[radeksimko]

The primary reason I raised this originally was that I thought this would make it easier to run against any Terraform version and do it quite fast via parallelised CI matrix, possibly with the help of setup-terraform.

You have a valid point about the local dev environment, but the same problem already exists for Go versions or variety of OS/arch combinations - none of which anyone will test locally except for that single Go version on single OS/arch they happen to have installed.