gitignore: Ignore test-driver

Automake (1.12 here) parallel-tests installs a test-driver file, another
file to add to .gitignore.

While at it, remove the duplicate cscope.out entry and add TAGS (the
result of automake's "make tag")

.gitignore

@@ -26,6 +26,7 @@ cscope.out
 /ltmain.sh
 /missing
 /stamp-h1
+/test-driver
 Makefile
 Makefile.in
-cscope.out
+TAGS