slice plugin unit test clang compiler warning fix

[traeak]

slice plugin unit test correct std::array initialization to use double braces

to test:

• with clang make check should not have compiler warnings for the slice unit tests.
• make check should also still pass.

[randall]

[approve ci autest]

[zwoop]

Doh, I already merged another PR with the same change (I think), closing this for now, reopen / repost if that other fix doesn't work.