Can I display asciidoc document in github?

Your file isn't syntactically valid (or rather the line is treated as plain text).

Your original file:

==This is a test
this is a test of using asciidoc

There's no space after the ==; I believe it's required.

A modified version that displays correctly:

== This is a test
this is a test of using asciidoc

See test.adoc and test2.adoc in this Gist.

The syntax for this feature, "One line titles", is documented in the Asciidoc user guide, section 11.2.

As a supplement to the accepted answer, Github (or at least the Wiki portion of Github) only accepts the .asciidoc suffix. .adoc and .ad don't seem to work. I only came upon this because I tried to manually change the pages in the wiki from .md to .ad and they wouldn't render, so check this if your problem persists.