test-alias: adapt for Windows
The test used 'echo' to test '!' style aliases. On Windows 'echo' is handled
by cmd and thus behaves very differently from the 'normal' echo command.
The simple workaround used here for using the same alias on all platforms
is to use 'printf' instead. Msys 'printf' will also handle sh quoting and
escaping in cmd.
Environment variable expansion with sh syntax is handled by launching sh.