Add static_cast<uint8_t> in HpackVarintEncoderTest.
Add static_cast<uint8_t> in HpackVarintEncoderTest.

This is because Chromium's std::string is based on signed char, so without this
change tests fail since https://quiche-review.googlesource.com/c/quiche/+/6000.

I have verified locally that HpackVarintEncoderTest.Short and .LastByteIsZero fail on Chromium without this patch but they pass with.

gfe-relnote: n/a, test-only change.
PiperOrigin-RevId: 254955501
Change-Id: I9cf5413b1de05fec86698ef6cdab6662fa44ead4

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