pw_unit_test: pw_log unit test event handler

This adds a pw_log based unit test event handler, and switches it to be
the default.

Change-Id: I83808e1e0ffcb0061644a45d4acc1d47f1161f57
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index cd9e2fe..58d3037 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -61,7 +61,7 @@
 pw_size_report_toolchains = []
 
 # Implementation of a main function for "pw_test" unit test binaries.
-pw_unit_test_main = "$dir_pw_unit_test:main"
+pw_unit_test_main = "$dir_pw_unit_test:simple_printing_main"
 
 # Path to a test runner to automatically run unit tests after they are built.
 #