NetEQ RTP Play: Optionally write output audio file

This CL makes the output audio file optional to more
quickly run neteq_rtpplay when no audio output is needed.
The CL also includes necessary adaptations because of pre-existing
dependencies (e.g., the output audio file name is used to create
the plotting script file names).

The command line arguments are retro-compatible - i.e., same behavior
when specifying the output audio file and the new flag
--output_files_base_name is not used.

This CL also includes a test script with which the retro-compatibility
has been verified.

Bug: webrtc:10337
Change-Id: Ie3f301b3b2ed0682fb74426d9cf452396f2b112b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/126224
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27067}

modules/audio_coding/BUILD.gn

@@ -1462,8 +1462,11 @@ if (rtc_include_tests) {
         ":neteq_test_factory",
         ":neteq_test_tools",
         "../../rtc_base:rtc_base_approved",
+        "../../rtc_base:stringutils",
         "../../system_wrappers:field_trial",
         "../../test:field_trial",
+        "//third_party/abseil-cpp/absl/strings",
+        "//third_party/abseil-cpp/absl/types:optional",
       ]
       sources = [
         "neteq/tools/neteq_rtpplay.cc",