bootstrap jimsh: pass all tests

In order to test bootstrap jimsh, it is very helpful
if it can pass all the unit tests.

- Set tcl_platform(bootstrap) to 1 for bootstrap jimsh or 0 otherwise
- Use getref to determine in we have references, not ref since we
  implement a poor-man's ref for bootstrap jimsh
- bootstrap jimsh package doesn't return a "Can't load package"
  message if loading the package fails
- exec tests using [open |command] need pipe
- bootstrap jimsh can't set file times with [file mtime]

Signed-off-by: Steve Bennett <steveb@workware.net.au>