ecc/tests: rename contiki processes names

Both process in 'testecc.c' and 'testfield.c' have the same name which prevents
linking both in the same application.

In testecc.c change process name to 'ecc_test'.
In testfield.c fix typo in the process name 'ecc_filed_test' to 'ecc_field_tset'.

Change-Id: Iaf13c376063a3dbabe480fdcf4de8ddc12b3f8fa
Signed-off-by: Gaëtan Harter <gaetan.harter@inria.fr>