[ELF] Fix broken LTO cache test NFC
[ELF] Fix broken LTO cache test NFC

Commit r318397 fixed the cache pruning interval which broke this test
as it was assuming that the cache pruning was always being
performed. Explicitly set prune interval to 0s to ensure this.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@318426 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld