core: Misc. updates to the "plan" context tests

Mostly this is about updating ctx.Plan callers to expect diags instead of
err, but also includes a few light updates to test fixtures, and a fix to
testModuleInline.

terraform/terraform_test.go

@@ -146,7 +146,9 @@ func testModuleInline(t *testing.T, sources map[string]string) *configs.Config {
 	// sources only this ultimately just records all of the module paths
 	// in a JSON file so that we can load them below.
 	diags := loader.InstallModules(cfgPath, true, configload.InstallHooksImpl{})
-	t.Fatal(diags.Error())
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
 
 	config, diags := loader.LoadConfig(cfgPath)
 	if diags.HasErrors() {

terraform/test-fixtures/plan-cdb-depends-datasource/main.tf

@@ -1,7 +1,7 @@
 resource "aws_instance" "foo" {
   count   = 2
   num     = "2"
-  compute = "${element(data.aws_vpc.bar.*.id, count.index)}"
+  compute = data.aws_vpc.bar[count.index].id
 
   lifecycle {
     create_before_destroy = true

terraform/test-fixtures/plan-module-variable-from-splat/main.tf

@@ -1,9 +1,9 @@
 module "mod1" {
-  source = "mod"
+  source = "./mod"
   param  = ["this", "one", "works"]
 }
 
 module "mod2" {
-  source = "mod"
+  source = "./mod"
   param  = ["${module.mod1.out_from_splat[0]}"]
 }

terraform/test-fixtures/plan-untargeted-resource-output/mod/main.tf

@@ -1,5 +1,5 @@
 locals {
-  "one" = 1
+  one = 1
 }
 
 resource "aws_instance" "a" {