tools/test_data, one .sha256 for each binary blob

//test/data contains a bunch of binary files (example traces,
UI baseline screenshots) that are update quite frequently,
hence unsuitable to be checked into the git repo.
Prior to this CL //test/data was pulled as an atomic .zip
archive, like compiler toolchains. This was quite inconvenient
as screenshots are updated quite frequently and the process
of updating the .zip archive is error prone.
This CL introduces a new layout based on one .sha256 file
for each binary file.
Those files are maintained by a script, tools/test_data,
which can be used as follows:

$ tools/test_data status
modified       : test/data/counters.json
needs upload   : test/data/foo
needs download : test/data/bar

$ tools/test_data download [--overwrite]
... downloads all new files.

$ tools/test_data upload
... uploads new and modified files.

Change-Id: Id052f6eab08b1c89911fe7e184a73b26cce3cc2d
diff --git a/test/data/chrome_android_systrace.pftrace.sha256 b/test/data/chrome_android_systrace.pftrace.sha256
new file mode 100644
index 0000000..63f5b7d
--- /dev/null
+++ b/test/data/chrome_android_systrace.pftrace.sha256
@@ -0,0 +1 @@
+a96005e5ee9059b89347573f7a7ca53097244fb3532956e0a62897ad77be2ddf
\ No newline at end of file