Rename the test files of shared helpers

The test files that hold no tests and only helpers for other test files
get names that end in _helper_test.go and say what they hold: the sync
buffer, the clock, and the summarizer. A reader then finds a shared
helper from the file name and does not look for tests in these files.
The content of the files does not change.

Issue: 947bddb

diff --git a/clock_test.go b/clock_helper_test.go
similarity index 100%
rename from clock_test.go
rename to clock_helper_test.go

diff --git a/summarizer_test.go b/summarizer_helper_test.go
similarity index 100%
rename from summarizer_test.go
rename to summarizer_helper_test.go

diff --git a/buffer_test.go b/sync_buffer_helper_test.go
similarity index 100%
rename from buffer_test.go
rename to sync_buffer_helper_test.go