Fix FecTest.FlexfecTest flakiness caused by seq. num. wraparound.

The CL in https://codereview.webrtc.org/2918333002/ enabled
FecTest.FlexfecTest and also added a sequence number offset between
the FEC packets and the media packets. This was to simulate that the
sequence numbers were generated from different spaces, i.e., that they
belong to different SSRCs.

The test does not account for sequence number wraparound, which means
that it could fail when the sequence number offset realization was large.
This CL fixes the problem by ensuring that the offset always lies in
[0, 2^15].

This CL also fixes spelling of UlpfecTest.

BUG=webrtc:7912
TESTED=ninja -C out/Debug && third_party/gtest-parallel/gtest-parallel --gtest_filter="*Flexfec*" -r 1000 out/Debug/modules_tests

Review-Url: https://codereview.webrtc.org/2966753002
Cr-Commit-Position: refs/heads/master@{#18863}
diff --git a/webrtc/modules/rtp_rtcp/test/testFec/test_fec.cc b/webrtc/modules/rtp_rtcp/test/testFec/test_fec.cc
index a65e8a6..b23a5f5 100644
--- a/webrtc/modules/rtp_rtcp/test/testFec/test_fec.cc
+++ b/webrtc/modules/rtp_rtcp/test/testFec/test_fec.cc
@@ -135,7 +135,7 @@
   uint16_t fec_seq_num_offset;
   if (use_flexfec) {
     fec_ssrc = random.Rand(1u, 0xfffffffe);
-    fec_seq_num_offset = random.Rand<uint16_t>();
+    fec_seq_num_offset = random.Rand(0, 1 << 15);
   } else {
     fec_ssrc = media_ssrc;
     fec_seq_num_offset = 0;
@@ -465,13 +465,13 @@
 
 // Too slow to finish before timeout on iOS. See webrtc:4755.
 #if defined(WEBRTC_IOS)
-#define MAYBE_UlpecTest DISABLED_UlpecTest
+#define MAYBE_UlpfecTest DISABLED_UlpfecTest
 #define MAYBE_FlexfecTest DISABLED_FlexfecTest
 #else
-#define MAYBE_UlpecTest UlpecTest
+#define MAYBE_UlpfecTest UlpfecTest
 #define MAYBE_FlexfecTest FlexfecTest
 #endif
-TEST(FecTest, MAYBE_UlpecTest) {
+TEST(FecTest, MAYBE_UlpfecTest) {
   RunTest(false);
 }