Delete test/constants.h

It's not possible to use constants.h for all RTP extensions
after the number of extensions exceeds 14, which is the maximum
number of one-byte RTP extensions. This is because some extensions
would have to be assigned a number greater than 14, even if the
test only involves 14 extensions or less.

For uniformity's sake, this CL also edits some files to use an
enum as the files involved in this CL, rather than free-floating
const-ints.

Bug: webrtc:10288
Change-Id: Ib5e58ad72c4d3756f4c4f6521f140ec59617f3f5
Reviewed-on: https://webrtc-review.googlesource.com/c/123048
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26728}

modules/rtp_rtcp/source/rtp_sender_audio_unittest.cc

@@ -22,7 +22,10 @@
 namespace webrtc {
 
 namespace {
-const int kAudioLevelExtensionId = 9;
+enum : int {  // The first valid value is 1.
+  kAudioLevelExtensionId = 1,
+};
+
 const uint16_t kSeqNum = 33;
 const uint32_t kSsrc = 725242;
 const uint8_t kAudioLevel = 0x5a;