Next batch of test-tree-cleaning changes

Next batch of test-tree-cleaning changes

Summary:
The changes here fall into several categories.

- some tests were redirecting inferior stdout/err to a file. For these I
  make sure we use an absolute path for the file. I also create a
  lldbutil.read_file_on_target helper function to encapsulate the
  differences between reading a file locally and remotely.
- some tests were redirecting the pexpect I/O into a file. For these I
  use a python StringIO object to avoid creating a file altogether.
- the TestSettings inferior was creating a file. Here, I make sure the
  inferior is launched with pwd=build-dir so that the files end up
  created there.
- lldb-mi --log (used by some tests) creates a log file in PWD without
  the ability say differently. To make this work I make sure to run
  lldb-mi with PWD=build_dir. This in turn necessitated a couple of
  changes in other lldb-mi tests, which were using relative paths to
  access the source tree.

Reviewers: aprantl

Subscribers: ki.stfu, mehdi_amini, lldb-commits

Differential Revision: https://reviews.llvm.org/D44159

llvm-svn: 327625