Improve shell testing.

Add some new control variables:
- AV1_ENCODE_CPU_USED: defaults to 1, allows control of
  cpu-used param passed to aomenc from the shell.
- AV1_ENCODE_TEST_FRAME_LIMIT: allows control of test
  frame limit from the shell.
- AOM_TEST_OUTPUT_DIR: support shell control of output
  dir.
- AOM_TEST_PRESERVE_OUTPUT: disables clean up of the
  test output directory when set to "yes".

Add default file names for IVF, OBU, and WebM test files:
- AV1_IVF_FILE
- AV1_OBU_ANNEXB_FILE
- AV1_OBU_SEC5_FILE
- AV1_WEBM_FILE

The above can be manipulated via the shell at run time.

More:
- Add OBU testing (encode/decode of annex-B and section-5).
- Use DISABLED_ prefix w/decode_with_drops to allow use of
  --run-disabled-tests.
- Explicitly disable decode_to_md5.
- Reverse test listing order in examples.sh to ensure
  that encoder tests run first.
- Move multithread decode test into a specific place
  (aomdec_av1_ivf_mulithread), and make other tests single threaded.
- Use aomenc_encode_test_fast_params() in the encode wrapper
  function used throughout the tests (encode_yuv_raw_input_av1()).

Change-Id: I85d5f11b116e708d5934690f99fec2377835e320
diff --git a/test/examples.sh b/test/examples.sh
index 1a8b4d3..367fce5 100755
--- a/test/examples.sh
+++ b/test/examples.sh
@@ -12,7 +12,7 @@
 ##
 . $(dirname $0)/tools_common.sh
 
-example_tests=$(ls $(dirname $0)/*.sh)
+example_tests=$(ls -r $(dirname $0)/*.sh)
 
 # List of script names to exclude.
 exclude_list="best_encode decode_to_md5 examples run_encodes tools_common"