examples.sh: Run all example tests.

- Remove vpxdec and vpxenc from the exclude list.
- vpx{dec,enc}.sh: Updates to support finding their executable when
  LIBVPX_BIN_PATH is setup for the examples.
- tools_common.sh: New library function, vpx_tool_path(). Provides
  support for finding the exectuables in vpx{dec,enc}.sh.

Change-Id: I730f11cceb44646491a7a7ff58603a4a760129ef
diff --git a/test/examples.sh b/test/examples.sh
index 7ba9cce..39f7e39 100755
--- a/test/examples.sh
+++ b/test/examples.sh
@@ -15,7 +15,7 @@
 example_tests=$(ls $(dirname $0)/*.sh)
 
 # List of script names to exclude.
-exclude_list="examples vpxdec vpxenc tools_common"
+exclude_list="examples tools_common"
 
 # Filter out the scripts in $exclude_list.
 for word in ${exclude_list}; do