Avoid adding tests unless they are configured
Avoid adding tests unless they are configured

Currently, test/encode_perf_test.cc, test/decode_perf_test.cc,
encoder sources and decoder sources are added to libaom_test_srcs.txt
without consideration of the configuration. Fixing this
addresses test failures in google3.

Change-Id: Id2e4c44574bc82baed088e3ab3a50597f9bed08c

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom