Allow the user to chose the verbosity at runtime

By default, cobertura-instrumented programs output 4 lines of informative
messages. In some testsuites, these lines can make the tests fail. Allow
the user to disable them by setting an environment variable before
running the testsuite.

We introduce a minimalistic trace system that reads the environment
variable. Currently, the trace level is indeed a yes/no configuration,
but we leave room for future finer-grained control with the same
environment variable.