testlib: Improve the silent logging mode

testlib: Improve the silent logging mode

Previously the silent logging mode suppressed passes, skips and internal
testlib info messages, but did not suppress debugging output, making it
hard to see the fails in a noisy test.  This commit changes silent mode
so that it suppresses all output except test failures and fatal errors,
making silent mode truly useful for seeing just the important test
output.

This commit also adds a selftest to verify the behaviour of silent mode.

Change-Id: I75420aead03682306210746a87e2a3b608b58fc6
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>