cmd/protoc-gen-go: add test data for proto3 extensions

Change-Id: I2d47db2c177553dc2a3dacc66c70db579c1145fd
Reviewed-on: https://go-review.googlesource.com/136535
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/test.bash b/test.bash
index d6b8440..23fcbdc 100755
--- a/test.bash
+++ b/test.bash
@@ -31,6 +31,10 @@
 fi
 register_binary conformance-test-runner $PROTOBUF_DIR/conformance/conformance-test-runner
 register_binary protoc $PROTOBUF_DIR/src/protoc
+# Allow protoc to find google/protobuf/*.proto.
+rm -rf $PROTOBUF_DIR/src/include
+mkdir -p $PROTOBUF_DIR/src/include
+ln -s ../google $PROTOBUF_DIR/src/include/google
 
 # Download each Go toolchain version.
 GO_LATEST=go1.11