Add SDK provider configuration tests, part 2 (GoogleCloudPlatform#7723)

* Add tests for `providerConfigure` testing `billing_project` config

* Fix `billing_project` test

* Add tests for `providerConfigure` testing `region` config

* Add tests for `providerConfigure` testing `zone` config

* Add tests for `providerConfigure` testing `user_project_override` config

* Remove old redundant `TestHandleSDKDefaults_*` tests

* Update code following creation of `transport` package

* Reposition `transport_tpg` in imports

* Add WIP of scopes test

* Fix defect in test case

* Fix scopes test

* Implement test cases for user config including zero values, and update tests to asset when field should be unset but is found in config

* Make test error messages clearer w.r.t provider config from user versus derived Config struct

* Fix import issue after rebase

* Add comments separating types of test case, add cases for empty strings overwritten by ENVs

* Rebase PR to pull in changes, including service package split

* Add test cases that show empty strings are ignored and ENVs are used instead

* Update text in test failure messages, add comments to signpost different types of test case

* Make lack of error more explicit in test case

* Fix import issues from rebasing branch

* update use of `ioutil` to `os`

* Make imports match what's on main

* Update `testFakeCredentialsPath` to `transport_tpg.TestFakeCredentialsPath`

* Add missing comma

* Move file into `provider_test` package, update imports and remove duplicate code

* Fix defect when trying to access `CredentialsFromJSON` function

* Add missing function calls to set ENVs used in tests

* Update tests for region field, add test case for use of self links

* Aad test case for zone field, self links are not shortened and usable

* Remove test case; `ConflictsWith` on fields not testable at this level

* Update test case names, add comments

mmv1/third_party/terraform/provider/provider.go.erb

@@ -62,7 +62,7 @@ func Provider() *schema.Provider {
 			"credentials": {
 				Type:     schema.TypeString,
 				Optional: true,
-				ValidateFunc: validateCredentials,
+				ValidateFunc: ValidateCredentials,
 				ConflictsWith: []string{"access_token"},
 			},
 
@@ -183,7 +183,7 @@ func Provider() *schema.Provider {
 	}
 
 	provider.ConfigureContextFunc = func(ctx context.Context, d *schema.ResourceData) (interface{}, diag.Diagnostics) {
-		return providerConfigure(ctx, d, provider)
+		return ProviderConfigure(ctx, d, provider)
 	}
 
 	transport_tpg.ConfigureDCLProvider(provider)
@@ -632,7 +632,7 @@ end     # products.each do
 		)
 }
 
-func providerConfigure(ctx context.Context, d *schema.ResourceData, p *schema.Provider) (interface{}, diag.Diagnostics) {
+func ProviderConfigure(ctx context.Context, d *schema.ResourceData, p *schema.Provider) (interface{}, diag.Diagnostics) {
 	err := transport_tpg.HandleSDKDefaults(d)
 	if err != nil {
 		return nil, diag.FromErr(err)
@@ -758,7 +758,7 @@ func providerConfigure(ctx context.Context, d *schema.ResourceData, p *schema.Pr
 	return transport_tpg.ProviderDCLConfigure(d, &config), nil
 }
 
-func validateCredentials(v interface{}, k string) (warnings []string, errors []error) {
+func ValidateCredentials(v interface{}, k string) (warnings []string, errors []error) {
 	if v == nil || v.(string) == "" {
 		return
 	}