testimage.bbclass: move codes into testimage_main

testimage.bbclass: move codes into testimage_main

testimage-auto is expected to run testimage task's codes automatically.
But in fact, it's currently missing some codes, including testimage_sanity
and create_rpm_index.

This leads to the problem of unexpected runtime failure of test_dnf_makecache.
The error message is as below.

  RESULTS - dnf.DnfRepoTest.test_dnf_makecache - Testcase 1744: ERROR

This error is caused by the fact that create_rpm_index is not executed
before running the tests.

There's no reason why such codes should not be in testimage_main, so
move them into it.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>