Replace use of ASSERT in test code.

In top level test functions, replaced with gtest ASSERT_*. In helper
methods in main test files, replaced with EXPECT_* or RTC_DCHECK on a
case-by-case basis.

In separate mock/fake classes used by tests (which might be of some
use also in tests of third-party applications), ASSERT was replaced
with RTC_CHECK, using

  git grep -l ' ASSERT(' | grep -v common.h | \
    xargs sed -i 's/ ASSERT(/ RTC_CHECK(/'

followed by additional includes of base/checks.h in affected files,
and git cl format.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2622413005
Cr-Commit-Position: refs/heads/master@{#16150}
diff --git a/webrtc/base/testutils.h b/webrtc/base/testutils.h
index 34b7606..332857d 100644
--- a/webrtc/base/testutils.h
+++ b/webrtc/base/testutils.h
@@ -28,6 +28,7 @@
 #include <vector>
 #include "webrtc/base/arraysize.h"
 #include "webrtc/base/asyncsocket.h"
+#include "webrtc/base/checks.h"
 #include "webrtc/base/common.h"
 #include "webrtc/base/gunit.h"
 #include "webrtc/base/nethelpers.h"
@@ -177,7 +178,7 @@
     va_start(args, format);
     char buffer[1024];
     size_t len = vsprintfn(buffer, sizeof(buffer), format, args);
-    ASSERT(len < sizeof(buffer) - 1);
+    RTC_CHECK(len < sizeof(buffer) - 1);
     va_end(args);
     QueueData(buffer, len);
   }
@@ -297,7 +298,7 @@
     va_start(args, format);
     char buffer[1024];
     size_t len = vsprintfn(buffer, sizeof(buffer), format, args);
-    ASSERT(len < sizeof(buffer) - 1);
+    RTC_CHECK(len < sizeof(buffer) - 1);
     va_end(args);
     QueueData(buffer, len);
   }