[llvm-objcopy] - Stop using Inputs/alloc-symtab.o

Initially Inputs/alloc-symtab.o was added in D42222.
It contains an allocatable .symtab section. Today
we are able to create such sections using yaml2obj.

Later people started using this input for no solid reason in their tests.
Now multiple of tests are using it. 
(And those tests do not need such a specific case actually).

In this patch I removed this binary and rewrote the few tests.

Differential revision: https://reviews.llvm.org/D65278

llvm-svn: 367310