[fpv, rv_timer] add fpv assertions to rv_timer_core_assert_fpv and rv_timer_interrupts_assert_fpv

### Description

`rv_timer_core_assert_fpv.sv` and `rv_timer_interrupts_assert_fpv.sv` currently only contain a module declaration and `// TODO: populate me with FPV`. I'd like to add assertions to both these modules.

Start by reading rv_timer_core_assert_fpv.sv and rv_timer_interrupts_assert_fpv.sv, which currently contain only module declarations and TODO markers. Determine the intended FPV assertions for both modules and verify that each TODO is replaced with validated assertions.

Written by the indexing model from the issue text.