arti-client: Only run BridgesConfig doc test if pt-client feature is enabled.

One of the tests run as part of cargo test -p arti-client fails because the pt-client feature is not enabled by default:

failures:

---- src/config.rs - config::BridgesConfig (line 221) stdout ----
error[E0432]: unresolved import `arti_client::config::pt`
 --> src/config.rs:225:26
  |
7 | use arti_client::config::pt::ManagedTransportConfigBuilder;
  |                          ^^ could not find `pt` in `config`

error[E0599]: no method named `bridges` found for mutable reference `&mut BridgesConfigBuilder` in the current scope
  --> src/config.rs:233:19
   |
15 | builder.bridges().bridges().push(bridge_1);
   |                   ^^^^^^^ private field, not a method

error[E0599]: no function or associated item named `default` found for struct `BridgeConfigBuilder` in the current scope
  --> src/config.rs:236:48
   |
18 | let mut bridge2_builder = BridgeConfigBuilder::default();
   |                                                ^^^^^^^ function or associated item not found in `BridgeConfigBuilder`

error[E0599]: no method named `bridges` found for mutable reference `&mut BridgesConfigBuilder` in the current scope
  --> src/config.rs:247:19
   |
29 | builder.bridges().bridges().push(bridge2_builder);
   |                   ^^^^^^^ private field, not a method

error[E0599]: no method named `transports` found for mutable reference `&mut BridgesConfigBuilder` in the current scope
  --> src/config.rs:255:19
   |
37 | builder.bridges().transports().push(transport);
   |                   ^^^^^^^^^^ method not found in `&mut BridgesConfigBuilder`

error: aborting due to 5 previous errors

Some errors have detailed explanations: E0432, E0599.
For more information about an error, try `rustc --explain E0432`.
Couldn't compile the test.

failures:
    src/config.rs - config::BridgesConfig (line 221)

test result: FAILED. 5 passed; 1 failed; 3 ignored; 0 measured; 0 filtered out; finished in 2.10s

This disables the BridgesConfig doc test if the pt-client feature is not enabled.

Closes #843 (closed)

Signed-off-by: Gabriela Moldovan gabi@torproject.org