QuicSpdyStreamTest HTTP/3 frame generation refactor.

Add HeadersFrame() and DataFrame() methods to QuicSpdyStreamTest that generate
HTTP/3 headers, and use them unless the test specifically needs to know the
frame header length or needs the frame header and frame payload separately.

Also minor fixes: factor out some string literals; fix progreesive_encoder typo;
restrict class member visibility; do not call Initialize if not necessary
(HasFrameHeader() now queries transport_version from GetParams() instead of
|connection_|).

gfe-relnote: n/a, test-only change.
PiperOrigin-RevId: 252088038
Change-Id: I28121b2a4994aafa1da84438cc4ff983d7a0586d