Tests: Teach RunCMake to ignore LLVMFlang -flang-experimental-exec warning

This flag is needed for now to tell `flang-new` to support creating
executables.  In our CI job we always pass this flag, but it is not
always used.  The compiler may warn:

    flang-new: warning: argument unused during compilation: '-flang-experimental-exec'

Teach RunCMake to drop such incidental lines before matching against
expected output.