[Tutorials] Fix uhi test timeouts in matplotlib.

On some Mac VMs, matplotlib.show() was entering an interactive event
loop, which caused four uhi tutorials to time out.
By switching matplotlib to the PNG backend, show() is skipped as follows:

  hist015_TH1_read_and_draw_uhi.py:88: UserWarning: FigureCanvasAgg is non-interactive, and thus cannot be shown
    plt.show()

Author: hageboeck

tutorials/CMakeLists.txt

@@ -37,11 +37,12 @@ else()                       # testing using an installation
   enable_testing()
 endif()
 
-# Set the environment for the tutorials, which is the eventual ROOT_environ
+# - Set the environment for the tutorials, which is the eventual ROOT_environ
 # plus some environment variables related to limiting the number of threads
 # used by NumPy.
 # See: https://stackoverflow.com/questions/30791550/limit-number-of-threads-in-numpy
-set(TUTORIAL_ENV ${ROOT_environ} OMP_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1)
+# - For matplotlib, disable a blocking event loop on show() using a non-interactive backend
+set(TUTORIAL_ENV ${ROOT_environ} OMP_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 MKL_NUM_THREADS=1 MPLBACKEND=AGG)
 
 #---Copy the CTestCustom.cmake file into the build directory--------
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/CTestCustom.cmake ${CMAKE_CURRENT_BINARY_DIR} COPYONLY)