Add tests for popen()/pclose() and dpopen()/dpclose() that work in
both shell (NSH) and no-shell (direct posix_spawnp) modes. The test
binary doubles as its own target via the --echo sub-command.
Tests:
1-4: popen - basic read, multi-arg, pclose, invalid mode
5-7: dpopen - basic read, multi-arg, dpclose
Signed-off-by: cuiziwei <cuiziwei@xiaomi.com>
