set_is_unit_test/2 will now correctly set options.

[stassa]

• Previously, set_is_unit_test/2 would only process the argument 'false' and leave options it sets unchanged. This was mainly a problem on windows where the option 'load' was left to 'show', causing the entire Metta $corelib to be echoed to the terminal while loading it to memory. The present commit fixes that.
• Part of oingoing work to address #227.

[stassa]

Slight clarification: the previous version of set_is_unit_test/2 (in metta_interp.pl) would only process the argument "false" when not running unit tests. When running unit tests it would presumably (haven't tested this) also accept "true". The problem is that it would not change any option other than 'test' -by passing it the value of its argument- and so it would leave the option 'load' set to 'show'. That 's what was responsible for dumping the $corelib to the terminal on loading... but only on windows as reported in trueagi-io/metta-wam#227. It is still not clear to me why the behaviour is different on windows and linux (WSL) so more work is needed but this begins to address #227.