Provide comprehensive testcases for various file removal modes.

* tests/Makefile.am: Add new testcases.
* tests/testsuite.at: Likewise.
* tests/incr09.at: Add description.
* tests/remfiles04a.at: New file.
* tests/remfiles05.at: Rename to ...
* tests/remfiles04b.at: ... this.
* tests/remfiles04.at: Rename to ...
* tests/remfiles04c.at: ... this.
* tests/remfiles05a.at: New file.
* tests/remfiles05b.at: New file.
* tests/remfiles06.at: Rename to ...
* tests/remfiles05c.at: ... this.
* tests/remfiles06a.at: New file.
* tests/remfiles06b.at: New file.
* tests/remfiles06c.at: New file.
* tests/remfiles07a.at: New file.
* tests/remfiles07b.at: New file.
* tests/remfiles07c.at: New file.
* tests/remfiles08a.at: New file.
* tests/remfiles08b.at: New file.
* tests/remfiles08c.at: New file.
* tests/remfiles08.at: Rename to ...
* tests/remfiles09a.at: ... this.
* tests/remfiles09b.at: New file.
* tests/remfiles07.at: Rename to ...
* tests/remfiles09c.at: ... this.