Upgrade tfexec to v0.18.1 + add tests for ExpectError

[austinvalle]

Background

A recent update of tfexecv0.18.0 caused a regression where detailed stderr information wasn't being passed to this testing module. This has now been fixed with the new release v0.18.1

Notes

• Added tests for ExpectError, so future CI can catch bugs similar to the one introduced in build(deps): Bump github.com/hashicorp/terraform-exec from 0.17.3 to 0.18.0 #58
  • I'm not sure how to cause the terraform refresh command to fail, so I left that test out for now
  • Example of new tests failing w/ tfexec v0.18.0
• Upgrading tfexec to v0.18.1 to fix bug of error not containing stderr information: bugfix: correctly populate stderr terraform-exec#372
• Removed t.Parallel() from a test that was causing me some problems on my local

[bendbennett]

LGTM 👍

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