update testing doc

[amtoine]

std testing has been deprecated -hidden to the users to be more precise- for quite some time now...

this PR updates the testing documentation page to not nudge people to have a look at that hiddent module, especially because downloading testing.nu manually and putting it anywhere is not a very good workflow :confused:

i removed all mentions to testing.nu and wrote a new section to run tests, which is split into two parts

• running tests from a Nupm package
• running tests for standalon modules or scripts

[fdncred]

Thanks