selftests: add missing gitignore files/dirs

Add missing files and directories to gitignore. Create .gitignore files
as needed.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/.gitignore

@@ -0,0 +1 @@
+kselftest

tools/testing/selftests/nsfs/.gitignore

@@ -0,0 +1,2 @@
+owner
+pidns

tools/testing/selftests/sigaltstack/.gitignore

@@ -0,0 +1 @@
+sas