Also add sweepers for TLS certificates and private keys to easily clean up resources leaked during any failed tests.

Terraform testing style guide seems to suggest camel case is used for
the main test name then an underscore separates different variations of
it.

Main changes were moving docs generation to tfplugindocs, and updating
the go-fastly SDK to v3. I added some changes to the upstream docs
generation to avoid having to globally install tfplugindocs. This was
also done upstream so I had to do some large merge conflict resolution
in this commit to combine the similar but different updates.

One commit message related to vendoring tfplugindocs was:

> Don't cache dependencies in github PR workflow, instead rely on /vendor
>
> Including the tfplugindocs module in vendor means it's updating with `go
> mod vendor` along with the other libraries used. When running `go
> install`, this vendored copy is used, and installed to a project-local
> /bin directory. This enables the version of tfplugindocs used to be
> independent of other go projects installed on one's system.
>
> This change means `make dependencies` is no longer used, and isn't
> needed in the github PR workflow. Furthermore, the source code for the
> tool is included in the /vendor already so the caching of ~/go/* isn't
> required either.

A couple naming/structure things resulting from different people writing the code. Have just tidied them up before PRing.

…#353)

* Add TLSCLientCert and TLSClientKey options for splunk logging

* Add some comments to clarify the usage splunk test tls cert values

* Update fastly/block_fastly_service_v1_splunk_test.go

* Update fastly/block_fastly_service_v1_splunk_test.go

* Update fastly/block_fastly_service_v1_splunk_test.go

* Update fastly/block_fastly_service_v1_splunk_test.go

* Update fastly/block_fastly_service_v1_splunk_test.go

* Update fastly/block_fastly_service_v1_splunk_test.go

* Update fastly/block_fastly_service_v1_splunk_test.go

Co-authored-by: Mark McDonnell <Integralist@users.noreply.github.com>

…custom

The intermediates_blob field of the fastly_tls_platform_certificate
resource can contain PEM blocks representing an arbitrary length chain
of certificates. The validation function for this field has been updated
to reflect this. It now loops through the provided string and checks
that each block it finds matches the expected block type until it
reaches the end of the string. Similarly the validation function for one
single block has been updated to fail if the string contains more than
one PEM block.

Was only used in creation function but should have also been used
in update too.

Also removed the `replace` directive in the go.mod to remove dependency
on opencredo fork.

- removal of unneeded .gitignore entry
- removal of superfluous whitespace in docs example block
- conversion of TypeList to TypeSet in plural data sources' `ids` field
- a couple typo fixes here and there
- removal of Set function for controlling set hashing, unneeded
- consolidation of function naming to include "Fastly" before resource
  name
- fix some acctest.RandomWithPrefix with duplicate prefix
- clarify some comments
- add some checks in testAcc.*Exists functions when accessing the map of
  resources in state to avoid a panic if resource not found

Add a -tfplugindocsPath command line argument to the parsing script to
make it a bit more robust than dynamically setting the PATH variable in
the Makefile. Defaults to local bin, as the Makefile expects, but I
still set the argument in the Makefile in case someone modifies the BIN
variable.