wasi-nn: Improve tests paths for local dev (#2309)

## Context

Path to models use `/assets` for testing inside docker. While testing directly from
the repo we are forced to use soft-links or modify the paths.

## Proposed Changes

Use relative path and adjust docker volumes in docs.

core/iwasm/libraries/wasi-nn/test/build.sh

@@ -7,7 +7,7 @@
     -Wl,--allow-undefined \
     -Wl,--strip-all,--no-entry \
     --sysroot=/opt/wasi-sdk/share/wasi-sysroot \
-    -I.. -I../src/utils \
+    -I../include -I../src/utils \
     -o test_tensorflow.wasm \
     test_tensorflow.c utils.c