Introduce IovecToStringPiece() in QuicStreamSequencerBufferTest.
Introduce IovecToStringPiece() in QuicStreamSequencerBufferTest.

Introduce IovecToStringPiece() helper method for brevity.  Also, previously
std::string() objects were instantiated at almost every call site, which
involves unnecessary copying.  Creating a QuicStringPiece works just as fine.

gfe-relnote: n/a, test-only change.
PiperOrigin-RevId: 254789421
Change-Id: I75036c8b9f9f5e7a42ef53790facf97b59d32d39

google/quiche,google/quiche,google/quiche,google/quiche