print path for diff test --keep-input option

[cphlipot1]

diff test framework has a --keep-input option to keep the generated
trace, however it is often in some difficult to discover file in /tmp
like /tmp/tmpyeoiui86

Print the path to make the location obvious to the user.

Before

$  tools/diff_test_trace_processor.py out/linux_clang_debug/trace_processor_shell --keep-input --name-filter="PowerPowerRails.power_rails"

...

[  FAILED  ] PowerPowerRails:power_rails_session_uuid
[ RUN      ] PowerPowerRails:power_rails_power_rails
[       OK ] PowerPowerRails:power_rails_power_rails (ingest: 114.00 ms query: 0.52 ms)
[ RUN      ] PowerPowerRails:power_rails_session_uuid_same_index_same_name
[       OK ] PowerPowerRails:power_rails_session_uuid_same_index_same_name (ingest: 97.09 ms query: 0.28 ms)

...

After

$  tools/diff_test_trace_processor.py out/linux_clang_debug/trace_processor_shell --keep-input --name-filter="PowerPowerRails.power_rails"

...

[  FAILED  ] PowerPowerRails:power_rails_session_uuid (trace_path: /tmp/tmpdgqttrtc)
[ RUN      ] PowerPowerRails:power_rails_session_uuid_same_index_same_name
[       OK ] PowerPowerRails:power_rails_session_uuid_same_index_same_name (trace_path: /tmp/tmpkbiznewm ingest: 92.65 ms query: 0.28 ms)
[ RUN      ] PowerPowerRails:power_rails_well_known_power_rails
[       OK ] PowerPowerRails:power_rails_well_known_power_rails (trace_path: /tmp/tmpn9secvgj ingest: 92.34 ms query: 0.28 ms)

...