This commit allows the MueLu TwoBillion test to compile and run
nontrivially again.

There were multiple problems:

 1) The TwoBillion test itself was using an Xpetra preprocessor macro
 that wasn't ever defined.
 2) The TwoBillion test was protected in CMake with an Xpetra variable
 that was set with "SET" instead of "SET_GLOBAL".
 3) MueLu wasn't using the ASSERT_DEFINED cmake macro, which throws an
 error if a variable isn't defined and would have caught issue #2.

Build/Test Cases Summary
Enabled Packages: MueLu, Xpetra
Enabled all Forward Packages
0) MPI_DEBUG => passed: passed=356,notpassed=0 (27.99 min)
1) SERIAL_RELEASE => passed: passed=348,notpassed=0 (15.15 min)
2) MPI_SS_DEBUG => FAILED: passed=619,notpassed=2 => Not ready to push! (54.96 min)
3) SERIAL_SS_RELEASE => passed: passed=354,notpassed=0 (26.55 min)
WARNING: Forced the push!