Move the contents of test/elf to test/ELF

Having two directories with the same name modulo case does not work well
on Windows. The new test/elf directory was added in r281108. I assume
the intention was to add the test file to the existing test/ELF dir.

llvm-svn: 281115

diff --git a/lld/test/elf/format-binary.test b/lld/test/ELF/format-binary.test
similarity index 100%
rename from lld/test/elf/format-binary.test
rename to lld/test/ELF/format-binary.test