Add an actions provider for testing Skylark rules.

The new provider gathers actions generated by any Skylark-based RuleConfiguredTarget, so long as the rule definition has _skylark_test=True set. For the moment this flag is under the user's control, but the intention is that it will be set by a test runner.

--
MOS_MIGRATED_REVID=134687396
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
index 0245ea6..2782bc89 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
@@ -114,7 +114,7 @@
                         callable.parameters().length == 0 || !callable.structField(),
                         "Method "
                             + method
-                            + " was annotated with both structField amd parameters.");
+                            + " was annotated with both structField and parameters.");
                     if (callable.parameters().length > 0 || callable.mandatoryPositionals() >= 0) {
                       int nbArgs =
                           callable.parameters().length