[Object/llvm-readobj] - Cleanup testing of the dynamic objects.

This patch touches a few test cases:

It removes dtflags.elf-x86-64 binary and elf-dtflags.test.
elf-dtflags.test is excessive because we have the
elf-dynamic-tags.test which test all non-machine specific tags.

It removes testing of --dynamic-table from test\Object\readobj-shared-object.test
(we have the elf-dynamic-tags.test for that), and simplifies this test case.

It moves testing of the headers from readobj-shared-object.test
to elf-file-headers.test.

Adds test/tools/llvm-readobj/elf-file-types.test and test/tools/llvm-readobj/elf-loadname.test.

It opens road for removing the readobj-shared-object.test completely soon.

Differential revision: https://reviews.llvm.org/D65068

llvm-svn: 366895