samples/static_lib: Add testcase.ini

Add testcase.ini file for qemu_x86.

We detected an unexpected behavior in sanitycheck before submitting
this patch:

  In testcase.ini, if platform_whitelist contains more than one
  platform only the last platform will work. So, here we just use
  qemu_x86.

  However, we believe this situation is caused by the structure of
  this sample. See mylib/Makefile.

Jira: ZEP-621

Change-Id: I656c1b9a69b93fd9f7b0d98a69f49b9641918a93
Signed-off-by: Flavio Santes <flavio.santes@intel.com>