[XRay] Fix fdr-thread-order.cc when current directory contains fdr-thread-order.cc

Summary:
Currently,

    cd test/xray/TestCases/Posix
    $build/bin/llvm-lit fdr-thread-order.cc

fails because `rm fdr-thread-order.*` deletes the .cc file.

This patch uses:

* %t as temporary directory name containing log files
* %t.exe as executable name

It does not delete %t after the test finishes for debugging convenience.
This matches the behavior of tests of various other LLVM components.

Log files will not clog up because the temporary directory (unique among
test files but the same among multiple invocations of a test) is cleaned
at the beginning of the test.

Reviewers: dberris, mboerger, eizan

Reviewed By: dberris

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D52638

llvm-svn: 343295