message: In 'make style', include headers in tests/unit, excluding the
  Catch2 header.
diff: @@ -710,10 +710,11 @@ ASTYLE_BIN = astyle
  ASTYLE = $(ASTYLE_BIN) --options=$(SRC)config/mfem.astylerc
  ASTYLE_VER = "Artistic Style Version 3.1"
  FORMAT_FILES = $(foreach dir,$(DIRS) $(EM_DIRS) config,$(dir)/*.?pp)
  -FORMAT_FILES += tests/unit/*.cpp
  +FORMAT_FILES += tests/unit/*.?pp
  UNIT_TESTS_SUBDIRS = general linalg mesh fem miniapps ceed
  FORMAT_FILES += $(foreach dir,$(UNIT_TESTS_SUBDIRS),tests/unit/$(dir)/*.?pp)
  -FORMAT_LIST = $(filter-out general/tinyxml2.cpp,$(wildcard $(FORMAT_FILES)))
  +FORMAT_EXCLUDE = general/tinyxml2.cpp tests/unit/catch.hpp
  +FORMAT_LIST = $(filter-out $(FORMAT_EXCLUDE),$(wildcard $(FORMAT_FILES)))
  COUT_CERR_FILES = $(foreach dir,$(DIRS),$(dir)/*.[ch]pp)
  COUT_CERR_EXCLUDE = '^general/error\.cpp' '^general/globals\.[ch]pp'
  