extract only the md5 + quote the result
fixes:
test/examples.sh: 47: local: img-176x144-0029.i420: bad variable name
Change-Id: I81c6a83c8a4e792a520fd7046c8eedcbd4af9a0c
Specifying the --prefix command line arg executes all test programs within the
context of the prefix string, which is assigned to VPX_TEST_PREFIX.
All test functions updated to include VPX_TEST_PREFIX in their eval command.
Change-Id: I2e215cc8f216048edf3269db02a6b5660fe32318