%merge: test support for api.value.type=union

* tests/glr-regression.at: here.

tests/README.md

@@ -0,0 +1,26 @@
+# AT_BISON_OPTION_PUSHDEFS/AT_BISON_OPTION_POPDEFS
+
+These two macros must be given the (main) directives so that they
+define a number of other macros to:
+- what the language is (so what compiler tool chain to use, what
+  definition of yylex/yyerror, etc.)
+- what skeleton is used
+
+# Keywords
+- action
+- api.value.type
+- c++
+- cex: counterexamples.
+- d: d language
+- deprec: deprecated features.
+- diagnostics
+- glr
+- graph
+- java
+- lac
+- %merge
+- multistart
+- push
+- report
+- %union
+- variant