`cargo afl test` fails linking when encountering doctests

[SingingTree]

When running cargo afl test doctests result in linking errors. Steps to reproduce:

1. (Work around Troubleshooting: linking with cc failed #141) export RUSTFLAGS=-Clink-arg=-fuse-ld=gold
2. Create a minimal lib: cargo new --lib afl_rs_doctest
3. Set contents of lib.rs to:

//! # Examples
//! ```
//! extern crate afl_rs_doctest;
//! assert_eq!(afl_rs_doctest::foo(), 1);
//! ```


pub fn foo() -> u8 {
    0x01
}

4. Try and test with afl: cargo afl test

Results in:

b@b-virtual-machine ~/projects/afl_rs_doctest $ cargo afl test
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s                                                                                                                                                 
     Running target/debug/deps/afl_rs_doctest-752d43b7c98a3a98

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

   Doc-tests afl_rs_doctest

running 1 test
test src/lib.rs -  (line 2) ... FAILED

failures:

---- src/lib.rs -  (line 2) stdout ----
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-L" "/home/b/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.0.rcgu.o" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.1.rcgu.o" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.2.rcgu.o" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.3.rcgu.o" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.4.rcgu.o" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.5.rcgu.o" "/tmp/rustdoctestieMm5E/rust_out.rust_out.7rcbfp3g-cgu.6.rcgu.o" "-o" "/tmp/rustdoctestieMm5E/rust_out" "-Wl,--gc-sections" "-pie" "-Wl,-zrelro" "-Wl,-znow" "-nodefaultlibs" "-L" "/home/b/projects/afl_rs_doctest/target/debug/deps" "-L" "/home/b/projects/afl_rs_doctest/target/debug/deps" "-L" "/home/b/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-Bstatic" "/home/b/projects/afl_rs_doctest/target/debug/deps/libafl_rs_doctest-4fc61169c7b196d3.rlib" "-Wl,--start-group" "-L" "/home/b/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-Bdynamic" "-lstd-a27f61c2a32117a5" "-Wl,--end-group" "-Wl,-Bstatic" "/home/b/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-f97f39378674ae7b.rlib" "-Wl,-Bdynamic" "-lafl-llvm-rt" "-ldl" "-lrt" "-lpthread" "-lpthread" "-lgcc_s" "-lc" "-lm" "-lrt" "-lpthread" "-lutil" "-lutil"
  = note: /usr/bin/ld: cannot find -lafl-llvm-rt
          collect2: error: ld returned 1 exit status
          

thread 'src/lib.rs -  (line 2)' panicked at 'couldn't compile the test', librustdoc/test.rs:333:13
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    src/lib.rs -  (line 2)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--doc'

[frewsxcv]

fixed in #143