Handle single file coverage with jacoco

[matbme]

Calling :Coverage on a jacoco report that convers a single file causes sign coverage to not appear. This happens due to the fact that somewhere down the line (not sure if it's neotest's parser or something else), single entries are "flattened". So, for example:

{ { a = "a" } }

becomes

{ a = "a" }

thus causing loops with ipairs to never run. This PR checks whether these elements have been flattened and wraps them in a single-entry table.