gitignore: Ignore default bsimt_bt test output folder

Similarly to sanitycheck the bsim_bt tests have their own
work folder where compilation output and coverage results are
kept.
Ignore the default folder

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>

.gitignore

@@ -30,6 +30,7 @@ doc/samples
 doc/latex
 doc/themes/zephyr-docs-theme
 sanity-out*
+bsim_bt_out
 scripts/grub
 doc/reference/kconfig/*.rst
 doc/doc.warnings