core: Fix TestContextImport_providerVarConfig

I updated the "Variables" map incorrectly in earlier commit 10fe50bbdb
while making bulk updates to get the tests compiling again with the
changed underlying APIs.

The original value here was "bar", incorrectly changed to "foo" in that
commit. Here we return it back to "bar".

terraform/context_import_test.go

@@ -323,7 +323,7 @@ func TestContextImport_providerVarConfig(t *testing.T) {
 		),
 		Variables: InputValues{
 			"foo": &InputValue{
-				Value:      cty.StringVal("foo"),
+				Value:      cty.StringVal("bar"),
 				SourceType: ValueFromCaller,
 			},
 		},
@@ -334,7 +334,7 @@ func TestContextImport_providerVarConfig(t *testing.T) {
 		configured = true
 
 		if v, ok := c.Get("foo"); !ok || v.(string) != "bar" {
-			return fmt.Errorf("bad value: %#v", v)
+			return fmt.Errorf("bad value %#v; want %#v", v, "bar")
 		}
 
 		return nil