[LLD][COFF] Avoid overwriting inputs in tests

Before this patch, these two tests were emitting both a .DLL and .LIB. The output .LIB file name also happens to be an input .LIB file name. This prevented the test from executing a second time when LLD is re-entrant (LLD_IN_TEST=2).

This is a support patch for https://reviews.llvm.org/D70378.