Added a --quiet option to capture test output.

This option will prevent messages from a test to be emitted unless
the test fails, in which case the full output will be shown.

This commit also cleans up messages from reporters, and unifies
error messaging.
Use correct ert-runner in tests.

Previously, the ert-runner binary used was the one installed from Melpa,
but we obviously want to test the local one.

Add {ERT-RUNNER} as a replacement for the path to the local binary.
Should see error, not output.
This is the output from the dot reporter, not ert.