cmd/internal/goobj: add test case for object file reader

Add test in which a input Go object file contains a very large number
of relocations (more than 1<<20).

Updates #41621.

Change-Id: If1ebf3c4fefbf55ddec4e05c5299e7c48fc697d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/278493


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>