Deliberate overriding of an existing file to tests

Deliberate overriding via `save_as` metadata should be allowed, even after the
overwrite detection feature. This commit is to add tests for deliberate
overriding. As a result, the relevant tests *should fail* after this commit.

Overriding feature introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72
Overwrite detection introduced by ff7410ce2ada85b486a67ae11874d60d135ff939
Delib. overriding and overwrite detect. Fixes #938

Make deliberate overriding (*) works with overwrites detection.

(*) first introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72

The following are decided to be deliberate override:
 - articles using the `save_as` metadata
 - pages using the `save_as` metadata
 - template pages (always)

Pelican now exits in the following 2 cases:
 - at least 2 not deliberate writes to the same file name (behaviour introduced
   by the overwrite detection feature ff7410ce2ada85b486a67ae11874d60d135ff939)
 - at least 2 deliberate writes to the same file name (new behaviour)

Also added info logging when deliberate overrides are performed.