fix: report the correct caller in assert panics

[immanuwell]

small fix, but this panic output was kinda lying.
assert.Assert(false) could report the outer frame instead of the real caller, so debugging was a bit meh.

Related: #1007

What changed

• switch src/assert.Assert to runtime.CallersFrames
• add a regression test for the reported file/function

Repro

1. create /tmp/assert_repro.go:

package main

import "github.com/envoyproxy/ratelimit/src/assert"

func fail() { assert.Assert(false) }
func main() { fail() }

2. run go run /tmp/assert_repro.go
3. before this change the panic points at main.main
4. after this change it points at main.fail, which is the real call site

Checks

• go test ./src/assert
• go test ./...