Fix include without context in macros

[kljnepk]

Fixes #2108

visit_Include currently uses yield from template._get_default_module()._body_stream
in the sync without context branch unconditionally.

That is fine for unbuffered output, but inside buffered frames such as macros it turns
the generated macro into a generator function, so rendering returns the generator repr
instead of the included template output.

This keeps the existing yield from path for unbuffered frames and switches buffered
frames to explicit iteration so included output is appended to the frame buffer instead.

Adds a regression test for {% include "included" without context %} inside a macro.