example tests: Make failures due to incorrect bin path easier to diagnose.

Add elog() to tools_common, and checks for the executable in each test.

Change-Id: I3c6334eca62c0b56040d91145abbe1bf5f9c763c
diff --git a/test/twopass_encoder.sh b/test/twopass_encoder.sh
index fe3cbbb..95d49d6 100755
--- a/test/twopass_encoder.sh
+++ b/test/twopass_encoder.sh
@@ -29,7 +29,10 @@
   local codec="$1"
   local output_file="${VPX_TEST_OUTPUT_DIR}/twopass_encoder_${codec}.ivf"
 
-  [ -x "${encoder}" ] || return 1
+  if [ ! -x "${encoder}" ]; then
+    elog "${encoder} does not exist or is not executable."
+    return 1
+  fi
 
   eval "${encoder}" "${codec}" "${YUV_RAW_INPUT_WIDTH}" \
       "${YUV_RAW_INPUT_HEIGHT}" "${YUV_RAW_INPUT}" "${output_file}" \