[LLD][ELF] Add test cases to improve code coverage of Thunks [NFC]

Add two test cases to improve the code coverage of ThunkSection creation
when there are no existing ThunkSections in range. There are two test
cases, one where a new section can be created and another to trigger the
"InputSection too large for range extension thunk" error message. A recent
code coverage report showed that this section of code wasn't covered by a
test case.

Differential Revision: https://reviews.llvm.org/D46261

llvm-svn: 331751