plainbox:exporter:text: make the test exporter more friendly

This patch changes the text exporter to behave better in practice, when
someone uses plainbox run without specifying any additional options. The
output is colorized (if applicable), and uses pretty, readable format
somewhat similar to more polished unit test runners.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>