meson: respect 'tests' build option in contrib

meson: respect 'tests' build option in contrib

Both the "netrc" credential helper and git-subtree(1) from "contrib/"
carry a couple of tests with them. These tests get wired up in Meson
unconditionally even in the case where `-Dtests=false`. As those tests
depend on the `test_enviroment` variable, which only gets defined in
case `-Dtests=true`, the result is an error:

```
$ meson setup -Dtests=false -Dcontrib=subtree build
[...]

contrib/subtree/meson.build:15:27: ERROR: Unknown variable "test_environment".
```

Fix the issue by not defining these tests at all in case the "tests"
option is set to `false`.

Reported-by: Sam James <sam@gentoo.org>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>