Makes it so that the value of the "go.version" property in
the output XML can be customized by the caller using the
"go-version" flag. If the flag is unspecified, falls back to
the previous behavior of using runtime.Version() as the value.
Fixes#48
When `go test` is run on multiple packages and with coverage collection
enabled, it appends coverage information to the final result line for
each package.
With this change, properly handle this additional information and add it
into the JUnit XML report as a property of the test suite.
* If a test log contains more than one package we need to wrap it
in a <testsuites> element in order to be valid xml. Bamboo
cannot read the file if it is not valid xml.
