COMP: Python tests should use VTK_TEMP_DIR for output

Tests should write their temporary files into VTK_TEMP_DIR. This patch
also adds a new function vtkGetTempDir() to vtk.util.misc.

This patch may solve a nmmber of mysterious test failures on some
builds.

Change-Id: I80a33383a9097a015c9f6ff635c49bcc8aac9f91
Merge topic 'UseTempDirForOutput1' into master

26cf0730 COMP: Python tests should use VTK_TEMP_DIR for output