Add OWNERS to euicc coretests directory

This directory previously relied on transitive owners through
core/tests/coretests, which was not granular enough. Adding the OWNERS
of the code directory under test.

Test: Adding owners file
Bug: 295006271
Change-Id: I360fc5c6681bd9ec4f0c8c29a32976aaaaef7c3a
1 file changed