test: support test.utils.command-exists on Windows

[compnerd]

cat is a common utility on Unix systems, however, it is not universally available. On Windows systems, resort to cmd (%COMSPEC%) as the shell interpreter is guaranteed to be present. It stands to reason that sh would be a viable alternative to cat as that is the default (POSIX) shell interpreter on Unix systems and should be present on any SuS or POSIX complaint system.