Move all NSM tests away from the `low' level

* lisp/net/nsm.el (network-security-level, nsm-tls-checks): Make
`low' a "check nothing" setting again, and move all the `low'
checks back to `medium'.  This makes the test suite work again.
1 job for master in 3 minutes and 30 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #3101


Name Stage Failure
test-all Test
Makefile:402: recipe for target 'xdisp.o' failed
make[2]: *** [xdisp.o] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/src'
Makefile:424: recipe for target 'src' failed
make[1]: *** [src] Error 2
make[1]: Leaving directory '/builds/emacs/emacs'
make: *** [bootstrap] Error 2
Makefile:1117: recipe for target 'bootstrap' failed
ERROR: Job failed: exit code 1