Go-Tfe support for creating test variables

[aaabdelgany]

Description

This PR adds support for consuming environment variables for remote terraform tests. It contains a full test suite.

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

go test -run TestTestVariables -v
=== RUN   TestTestVariablesList
=== RUN   TestTestVariablesList/without_list_options
=== RUN   TestTestVariablesList/empty_list_options
=== RUN   TestTestVariablesList/with_page_size
--- PASS: TestTestVariablesList (6.52s)
    --- PASS: TestTestVariablesList/without_list_options (0.22s)
    --- PASS: TestTestVariablesList/empty_list_options (0.20s)
    --- PASS: TestTestVariablesList/with_page_size (0.30s)
=== RUN   TestTestVariablesCreate
=== RUN   TestTestVariablesCreate/with_valid_options
=== RUN   TestTestVariablesCreate/when_options_has_an_empty_string_value
=== RUN   TestTestVariablesCreate/when_options_has_an_empty_string_description
=== RUN   TestTestVariablesCreate/when_options_has_a_too-long_description
=== RUN   TestTestVariablesCreate/when_options_is_missing_value
=== RUN   TestTestVariablesCreate/when_options_is_missing_key
=== RUN   TestTestVariablesCreate/when_options_has_an_empty_key
=== RUN   TestTestVariablesCreate/when_options_is_missing_category
--- PASS: TestTestVariablesCreate (5.68s)
    --- PASS: TestTestVariablesCreate/with_valid_options (0.31s)
    --- PASS: TestTestVariablesCreate/when_options_has_an_empty_string_value (0.22s)
    --- PASS: TestTestVariablesCreate/when_options_has_an_empty_string_description (0.23s)
    --- PASS: TestTestVariablesCreate/when_options_has_a_too-long_description (0.18s)
    --- PASS: TestTestVariablesCreate/when_options_is_missing_value (0.20s)
    --- PASS: TestTestVariablesCreate/when_options_is_missing_key (0.00s)
    --- PASS: TestTestVariablesCreate/when_options_has_an_empty_key (0.00s)
    --- PASS: TestTestVariablesCreate/when_options_is_missing_category (0.00s)
=== RUN   TestTestVariablesUpdate
=== RUN   TestTestVariablesUpdate/with_valid_options
=== RUN   TestTestVariablesUpdate/when_updating_a_subset_of_values
=== RUN   TestTestVariablesUpdate/with_sensitive_set
=== RUN   TestTestVariablesUpdate/without_any_changes
=== RUN   TestTestVariablesUpdate/with_invalid_variable_ID
--- PASS: TestTestVariablesUpdate (9.16s)
    --- PASS: TestTestVariablesUpdate/with_valid_options (0.21s)
    --- PASS: TestTestVariablesUpdate/when_updating_a_subset_of_values (0.25s)
    --- PASS: TestTestVariablesUpdate/with_sensitive_set (0.21s)
    --- PASS: TestTestVariablesUpdate/without_any_changes (2.84s)
    --- PASS: TestTestVariablesUpdate/with_invalid_variable_ID (0.00s)
=== RUN   TestTestVariablesDelete
=== RUN   TestTestVariablesDelete/with_valid_options
=== RUN   TestTestVariablesDelete/with_non_existing_variable_ID
=== RUN   TestTestVariablesDelete/with_invalid_variable_ID
--- PASS: TestTestVariablesDelete (5.28s)
    --- PASS: TestTestVariablesDelete/with_valid_options (0.29s)
    --- PASS: TestTestVariablesDelete/with_non_existing_variable_ID (0.18s)
    --- PASS: TestTestVariablesDelete/with_invalid_variable_ID (0.00s)
PASS
ok  	github.com/hashicorp/go-tfe	26.840s

[nfagerlund]

OK, this all seems pretty good to me, and I got those beta tests to pass locally.

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.