tests/periph_gpio_ll: drop core dump merged by accident

[maribu]

Contribution description

As the title says. It also adds a .gitignore in tests/ and examples/ that matches files named core in a direct subfolder of tests/ and examples/

Testing procedure

$ touch core/foo # should *NOT* be ignored
$ touch core/core # should *NOT* be ignored
$ touch drivers/core # should *NOT* be ignored
$ touch examples/hello-world/core # should be ignored
$ touch tests/unittests/core # should be ignored
$ git add .
$ git status
On branch core_dumps
Your branch is up to date with 'origin/core_dumps'.

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	new file:   core/core
	new file:   core/foo
	new file:   drivers/core

Issues/PRs references

#16787 (comment)

[maribu]

All green. I updated the .gitignore expressions to also match tests/*/core.* and examples/*/core.* except for core.c, core.h, core.txt and core.md since the ACK. If you agree, please hit merge :)