Fix accidentally-harmless thinko in psqlscan_test_variable().

Fix accidentally-harmless thinko in psqlscan_test_variable().

This code was passing literal strings to psqlscan_emit,
which is quite contrary to that function's specification:
"If you pass it something that is not part of the yytext
string, you are making a mistake".  It accidentally worked
anyway, even in non-safe_encoding mode.  psqlscan_emit
would compute a garbage "reference" pointer, but would
never dereference that since the passed string is all-ASCII.
So there's no live bug today, but that is a happenstance
outcome of psqlscan_emit's current implementation.

Let's make psqlscan_test_variable do what it's supposed to,
namely append directly to the output buffer.  This is just
future-proofing against possible changes in psqlscan_emit,
so I don't feel a need to back-patch.