Separated juju/osenv environment variable tests OS-specifically.

The juju/osenv environment variable tests are now separated into platform-specific files.

TestJujuHome had to be separated OS-specifically anyhow and TestBlankJujuHomeEnvVar was full of runtime.GOOS checks for every PatchEnvironment/Assert it had which made it look untidy and thus required separation as well. TestJujuHomeEnvVar was way too small for it to deserve its own file really, 3 lines of code duplication being a small price to pay for a cleaner looking project.

newline before the // +build tag