[lldb] Use file to synchronize TestDeepBundle and TestBundleWithDotInFilename

Currently these two tests use an arbitrary wait of 5 seconds for the
inferior to finish setting up. When the test machine is under heavy load
this sometimes is insufficient leading to spurious test failures. This
patch adds synchronization trough a token on the file system. In
addition to making the test more reliable it also makes it much faster
because we no longer have to wait the full 5 seconds if the setup was
completed faster than that.

Differential revision: https://reviews.llvm.org/D85915