Allow per-module terraform fixtures (#1914)

* Allow terraform fixtures for examples

* Allow defining multiple fixtures, and named fixtures under tests/fixtures/

* Enable e2e for wiktorn

* Fix prepare_files call for e2e

* Move fixture to separate file, fix test

* Revert shallow-copying symlinks, performane penalty - 20%

* Update tfdoc.py to list used fixtures

---------

Co-authored-by: Wiktor Niesiobędzki <wiktorn@google.com>

modules/net-lb-app-ext/README.md

@@ -31,6 +31,7 @@ Due to the complexity of the underlying resources, changes to the configuration
 - [Files](#files)
 - [Variables](#variables)
 - [Outputs](#outputs)
+- [Fixtures](#fixtures)
 <!-- END TOC -->
 
 ### Minimal HTTP Example
@@ -40,18 +41,18 @@ An HTTP load balancer with a backend service pointing to a GCE instance group:
 ```hcl
 module "glb-0" {
   source     = "./fabric/modules/net-lb-app-ext"
-  project_id = "myprj"
+  project_id = var.project_id
   name       = "glb-test-0"
   backend_service_configs = {
     default = {
       backends = [
-        { backend = "projects/myprj/zones/europe-west8-b/instanceGroups/myig-b" },
-        { backend = "projects/myprj/zones/europe-west8-c/instanceGroups/myig-c" },
+        { backend = module.compute-mig-a.group.id },
+        { backend = module.compute-mig-b.group.id },
       ]
     }
   }
 }
-# tftest modules=1 resources=5
+# tftest modules=3 resources=9 fixtures=fixtures/compute-mig-ab.tf e2e
 ```
 
 ### Minimal HTTPS examples
@@ -895,4 +896,8 @@ module "glb-0" {
 | [neg_ids](outputs.tf#L67) | Autogenerated network endpoint group ids. |  |
 | [psc_neg_ids](outputs.tf#L74) | Autogenerated PSC network endpoint group ids. |  |
 | [serverless_neg_ids](outputs.tf#L81) | Autogenerated serverless network endpoint group ids. |  |
+
+## Fixtures
+
+- [compute-mig-ab.tf](../../tests/fixtures/compute-mig-ab.tf)
 <!-- END TFDOC -->