Restore test coverage of LZ4Stream_gets().

Restore test coverage of LZ4Stream_gets().

In commit a45c78e32 I removed the only regression test case that
reaches this function, because it turns out that we only use it
if reading an LZ4-compressed blobs.toc file in a directory dump,
and that is a state that has to be created manually.  That seems
like a bad thing to not test, not so much for LZ4Stream_gets()
itself as because it means the squirrely eol_flag logic in
LZ4Stream_read_internal() is not tested.

The reason for the change was that I thought the lz4 program did not
have any way to perform compression without explicit specification
of the output file name.  However, it turns out that the syntax
synopsis in its man page is a lie, and if you read enough of the
man page you find out that with "-m" it will do what's needful.
So restore the manual compression step in that test case.

Noted while testing some proposed changes in pg_dump's compression
logic.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3515357.1760128017@sss.pgh.pa.us
Backpatch-through: 17