New test file 'memerr.lua'

Tests for memory-allocation errors moved from 'api.lua' to this new
file, as 'api.lua' was already too big. (Besides, these tests have
nothing to do with the API.)

testes/packtests

@@ -28,6 +28,7 @@ $NAME/literals.lua  \
 $NAME/locals.lua  \
 $NAME/main.lua  \
 $NAME/math.lua  \
+$NAME/memerr.lua  \
 $NAME/nextvar.lua  \
 $NAME/pm.lua  \
 $NAME/sort.lua  \