configurator: Use native directory separator

Although Windows APIs generally permit "/" or "\\" for directory
separators in paths, cmd.exe does not recognize "./" when invoking
executables using a relative path and prints the following error:

    '.' is not recognized as an internal or external command,
    operable program or batch file.

Therefore, use "\\" when invoking tests on Windows.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>