test: http2 util passing array in te header

This adds a test case in which array with two values is passed to
param value in isIllegalConnectionSpecificHeader

Refs: nodejs#14985

Author: trivikr

lib/internal/http2/util.js

@@ -360,8 +360,7 @@ function isIllegalConnectionSpecificHeader(name, value) {
     case HTTP2_HEADER_TRANSFER_ENCODING:
       return true;
     case HTTP2_HEADER_TE:
-      const val = Array.isArray(value) ? value.join(', ') : value;
-      return val !== 'trailers';
+      return value !== 'trailers';
     default:
       return false;
   }

test/parallel/test-http2-util-headers-list.js

@@ -266,5 +266,10 @@ common.expectsError({
   message: regex
 })(mapToHeaders({ [HTTP2_HEADER_TE]: ['abc'] }));
 
+common.expectsError({
+  code: 'ERR_HTTP2_INVALID_CONNECTION_HEADERS',
+  message: regex
+})(mapToHeaders({ [HTTP2_HEADER_TE]: ['abc', 'trailers'] }));
+
 assert(!(mapToHeaders({ te: 'trailers' }) instanceof Error));
 assert(!(mapToHeaders({ te: ['trailers'] }) instanceof Error));