test-tap: Fix Solaris /bin/sh "prove: not found" on stderr.

test-tap: Fix Solaris /bin/sh "prove: not found" on stderr.

The prove command is intended to be optional, just to display a summary
of the test result if it is available.  However, on old Solaris /bin/sh,
our command

  prove ... 2>&1

doesn't redirect the error message from the shell when prove is not
found.  We can use a command like like

  { prove ...; } 2>&1

instead, which does redirect the error.