Propagate reattach info

[kmoe]

Requires hashicorp/terraform-exec#78

This PR fixes issues introduced in #35, in which Terraform commands were reimplemented using terraform-exec. The tfexec paradigm does not permit transparent propagation of environment variables through to the Terraform CLI process, which meant that the crucial TF_PROVIDERS_REATTACH env var was not present in the binary testing environment. This behaviour, and the ability for the caller to set other env vars at arbitrary times, is restored by allowing WorkingDir to maintain a pseudo-environment.

Add wd.SetReattachInfo(reattachInfo string) and wd.UnsetReattachInfo()

The caller provides the reattach info (the contents of TF_PROVIDERS_REATTACH), and this is stored on WorkingDir and added to each tfexec command as necessary.

Restore previous behaviour of wd.Setenv(envVar, val string) (reverted, see comment below)

Since terraform-plugin-test is no longer in control of the environment variables for each Terraform command, we instead ensure the correct environment is set before each tfexec.Terraform command.

Note: wd.Setenv and wd.Unsetenv are not used in the SDK, and should be removed if there is no clear use case. This is a breaking change and so will be dealt with separately as https://github.com/hashicorp/terraform-plugin-test/issues/39.

[paddycarver]

Called out some errors that need checked (sorry for the repetition).

We could also automate this with https://github.com/kisielk/errcheck or golangci-lint.

[kmoe]

After some discussion, I'm removing the propagation of the whole os.Environ() into the tfexec environment, because it causes unexpected results with reattach tests. Leaving the two (now a little counterintuitive, and in any case unused) wd.Setenv() and wd.Unsetenv(), which will be removed when tftest is very shortly moved into the SDK itself.