Usage: squash_testlogs.py START_DIRECTORY OUTPUT_FILE

Example: squash_testlogs.py /tf/pkg/testlogs /tf/pkg/merged.xml

Recursively find all the JUnit test.xml files in one directory, and merge any
of them that contain failures into one file. The TensorFlow DevInfra team
uses this to generate a simple overview of an entire pip and nonpip test
invocation, since the normal logs that Bazel creates are too large for the
internal invocation viewer.