[llvm-objcopy][test] Clean up -B tests

-B is ignored for GNU objcopy compatibility after D67215/r371914.

* Delete mentions of -B from input-output-target.test - we have enough -B tests.
* Merge binary-input-with-arch.test into binary-output-target.test.

Reviewed By: rupprecht

Differential Revision: https://reviews.llvm.org/D67693

llvm-svn: 372809