libctf: add rewriting tests

Now there's a chance of it actually working, we can add more tests for
the long-broken dict read-and-rewrite cases.  This is the first ever
test for the (rarely-used, unpleasant, and until recently completely
broken) ctf_gzwrite function.

libctf/

	* testsuite/libctf-regression/gzrewrite*: New test.
	* testsuite/libctf-regression/zrewrite*: Likewise.