mtd: tests: rename sources in order to link a helper object

Each mtd test module have a single source whose name is the same as
the module name.  In order to link a single object including helper
functions to every test module, this rename these sources to the
different names.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Vikram Narayanan <vikram186@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>