sh tests: Add support for running tested programs within another program.

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
diff --git a/test/simple_encoder.sh b/test/simple_encoder.sh
index a0b0e13..c4a6280 100755
--- a/test/simple_encoder.sh
+++ b/test/simple_encoder.sh
@@ -34,7 +34,7 @@
     return 1
   fi
 
-  eval "${encoder}" "${codec}" "${YUV_RAW_INPUT_WIDTH}" \
+  eval "${VPX_TEST_PREFIX}" "${encoder}" "${codec}" "${YUV_RAW_INPUT_WIDTH}" \
       "${YUV_RAW_INPUT_HEIGHT}" "${YUV_RAW_INPUT}" "${output_file}" 9999 \
       ${devnull}