[llvm-exegesis] Refactor BenchmarkMeasure instantiation in tests

This patch refactors the instantiation of BenchmarkMeasure within all
the unit tests to use BenchmarkMeasure::Create rather than through
direct struct instantialization. This allows us to change what values
are stored in BenchmarkMeasure without getting compiler warnings on
every instantiation in the unit tests, and is also just a cleanup in
general as the Create function didn't seem to exist at the time the unit
tests were originally written.