fix a multiline comment warning

Summary:
DeterministicScheduleTest.cpp contained several single-line C++ comments that
ended in a trailing backslash.  This makes the preprocessor treat the following
line as a comment as well, even if it does not start with `//`.  Newer versions
of gcc emit warnings about this.

This changes the comment in DeterministicScheduleTest.cpp to use `/* */` to
avoid this issue.

Reviewed By: siyengar

Differential Revision: D6735672

fbshipit-source-id: 162c735507a643ce0dbee58f1f054865237e1eba