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

http2/hpack/varint/hpack_varint_encoder_test.cc

diff --git a/http2/hpack/varint/hpack_varint_encoder_test.cc b/http2/hpack/varint/hpack_varint_encoder_test.cc
index fa05948..94f9a9e 100644
--- a/http2/hpack/varint/hpack_varint_encoder_test.cc
+++ b/http2/hpack/varint/hpack_varint_encoder_test.cc
@@ -36,7 +36,8 @@
                                kShortTestData[i].prefix_length,
                                kShortTestData[i].value, &output);
     ASSERT_EQ(1u, output.size());
-    EXPECT_EQ(kShortTestData[i].expected_encoding, output[0]);
+    EXPECT_EQ(kShortTestData[i].expected_encoding,
+              static_cast<uint8_t>(output[0]));
   }
 }
 
@@ -136,7 +137,7 @@
                                kLastByteIsZeroTestData[i].value, &output);
     ASSERT_EQ(2u, output.size());
     EXPECT_EQ(kLastByteIsZeroTestData[i].expected_encoding_first_byte,
-              output[0]);
+              static_cast<uint8_t>(output[0]));
     EXPECT_EQ(0b00000000, output[1]);
   }
 }