src,test: fix gcc warning on comments

Fix Wcomment complain from gcc

../test/embedding/embedtest.cc:67:3: warning: multi-line comment [-Wcomment]
   67 |   // embedtest js_code_to_eval arg1 arg2... \
      |   ^
../test/embedding/embedtest.cc:72:3: warning: multi-line comment [-Wcomment]
   72 |   // embedtest --embedder-snapshot-blob blob-path \
      |