examples,tests: ignore core dumps via .gitignore

This will ignore files named `core` and `core.*` (except `core.c`,
`core.h`, `core.md`, `core.txt`) placed directly in the application
folder. This is where `make` is typically called and core dump due to
a crashing native application, flashing tool, or GDB would be placed.
The pattern is intentionally quite narrow, as there is e.g. a core
source folder that should still be monitored.

examples/.gitignore

@@ -0,0 +1,6 @@
+/*/core
+/*/core.*
+!/*/core.c
+!/*/core.h
+!/*/core.md
+!/*/core.txt

tests/.gitignore

@@ -0,0 +1,6 @@
+/*/core
+/*/core.*
+!/*/core.c
+!/*/core.h
+!/*/core.md
+!/*/core.txt