Add shell scripts for verifying basic vpx{dec,enc} features.

Tests the basics (first confirms feature is available in vpx_config.h):
- VP8 decode (in IVF file).
- VP9 decode (in WebM file).
- VP8 encode (to IVF and WebM).
- VP9 encode (to IVF and WebM).
- VP9 lossless encode (to IVF, currently disabled due to failure).
- Pipe input (to vpxdec and vpxenc).

Test data path and path to vpx{dec,enc} have been parameterized. In
addition:
- Supports disabling tests (test names prefixed with DISABLED_ are not
  run by default).
- Supports filtering tests.

vpxdec.sh: Tests vpxdec.
vpxenc.sh: Tests vpxenc.
tools_common.sh: Common test functions.

Change-Id: I0612c88b8dd6049a05bbbc79a317a0cca61733a5