Harden tableam against nonexistant / wrong kind of AMs.

Previously it was allowed to set default_table_access_method to an
empty string. That makes sense for default_tablespace, where that was
copied from, as it signals falling back to the database's default
tablespace. As there is no equivalent for table AMs, forbid that.

Also make sure to throw a usable error when creating a table using an
index AM, by using get_am_type_oid() to implement get_table_am_oid()
instead of a separate copy. Previously we'd error out only later, in
GetTableAmRoutine().

Thirdly remove GetTableAmRoutineByAmId() - it was only used in an
earlier version of 8586bf7ed8.

Add tests for the above (some for index AMs as well).

src/backend/commands/tablecmds.c

@@ -832,22 +832,9 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 			 relkind == RELKIND_MATVIEW)
 		accessMethod = default_table_access_method;
 
-	/*
-	 * look up the access method, verify it can handle the requested features
-	 */
+	/* look up the access method, verify it is for a table */
 	if (accessMethod != NULL)
-	{
-		HeapTuple	tuple;
-
-		tuple = SearchSysCache1(AMNAME, PointerGetDatum(accessMethod));
-		if (!HeapTupleIsValid(tuple))
-				ereport(ERROR,
-						(errcode(ERRCODE_UNDEFINED_OBJECT),
-						 errmsg("table access method \"%s\" does not exist",
-								 accessMethod)));
-		accessMethodId = ((Form_pg_am) GETSTRUCT(tuple))->oid;
-		ReleaseSysCache(tuple);
-	}
+		accessMethodId = get_table_am_oid(accessMethod, false);
 
 	/*
 	 * Create the relation.  Inherited defaults and constraints are passed in