char: misc: add test cases

Add test cases for static and dynamic minor number allocation and
deallocation.

While at it, improve description and test suite name.

Some of the cases include:

- that static and dynamic allocation reserved the expected minors.

- that registering duplicate minors or duplicate names will fail.

- that failing to create a sysfs file (due to duplicate names) will
  deallocate the dynamic minor correctly.

- that dynamic allocation does not allocate a minor number in the static
  range.

- that there are no collisions when mixing dynamic and static allocations.

- that opening devices with various minor device numbers work.

- that registering a static number in the dynamic range won't conflict with
  a dynamic allocation.

This last test verifies the bug fixed by commit 6d04d2b ("misc:
misc_minor_alloc to use ida for all dynamic/misc dynamic minors") has not
regressed.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Link: https://lore.kernel.org/r/20250423-misc-dynrange-v4-2-133b5ae4ca18@igalia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Thadeu Lima de Souza Cascardo