Work around Msys weakness in Testlib.pm's command_like()

Work around Msys weakness in Testlib.pm's command_like()

When output of IPC::Run::run () is redirected to scalar references, in
certain circumstances the Msys perl does not correctly detect that the
end of file has been seen, making the test hang indefinitely. One such
circumstance is when the command is 'pg_ctl start', and such a change
was made in commit f13ea95f9e. The workaround, which only applies on
MSys, is to redirect the output to temporary files and then read them in
when the process has finished.

Patch by me, reviewed and tweaked by Tom Lane.