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

Instead of maintaining a list of version numbers in [internal/testutil/tfcache.go](https://github.com/hashicorp/terraform-exec/blob/cb3a405d1356be591b81e07ac394ac5171b685ac/tfexec/internal/testutil/tfcache.go#L14), 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.:
https://github.com/hashicorp/terraform-exec/blob/cb3a405d1356be591b81e07ac394ac5171b685ac/tfexec/internal/e2etest/upgrade012_test.go#L14-L16

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

Start with internal/testutil/tfcache.go and the CircleCI configuration, then inspect runTest() and upgrade012_test.go for tests that depend on the default version list. Determine how the CI version matrix and TFEXEC_E2ETEST_VERSIONS should provide versions in CI and local development, while preserving behavior for tests that do not use runTest().

Written by the indexing model from the issue text.