Disable test output buffering when testing only one package.
Disable test output buffering when testing only one package.

Buffering is already disabled when -p=1, which allows to see test output
in real time. With this change it will be done automatically if only one
package matched.

gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs