unittest: Work around build failure on MSVC builders

MSVC insists on using the deleted move constructor instead of the copy
constructor:

http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/41203

C:\ps4-buildslave2\lld-x86_64-win7\llvm-project\llvm\unittests\ADT\CoalescingBitVectorTest.cpp(193):
error C2280: 'llvm::CoalescingBitVector<unsigned
int,16>::CoalescingBitVector(llvm::CoalescingBitVector<unsigned int,16>
&&)': attempting to reference a deleted function