add FuncPc for subtest

[Dinhar]

Added func pointer for subtests

Closes #4

[metafates]

Thanks! Minor issue: we still have such comment on this field:

// FuncPC is the program counter (PC) of this test function.
//
// NOTE: it may be empty (zero) in some cases, e.g. hooks and sub-tests.
// Use with caution.
FuncPC uintptr

Let's change it to:

-// NOTE: it may be empty (zero) in some cases, e.g. hooks and sub-tests.
+// NOTE: it may be empty (zero) in some cases, e.g. hooks.

Also, let's update CHANGELOG.md with

## Unreleased

### Added

- Provide FuncPC for sub-tests.

[metafates]

Thanks