[flang] Fix combined folding of FINDLOC/MAXLOC/MINLOC

The tests for folding these intrinsics neglected to name the
logical scalars with a leading "test_", so test failures caused
by recent work to implement a combined constant folding facility
for these intrinsics wasn't catching some bugs.  This patch fixes
the tests and the bugs.

Differential Revision: https://reviews.llvm.org/D112741