Fix failing unit tests to successfully run and test the rustc bootstrap build.

[Yashinde145]

After building and testing rustc bootstrap build via cmd python3 src/bootstrap/bootstrap.py test --exclude<some excluded test cases list> --no-fail-fast --bless --target x86_64-poky-linux-gnu, some unit test fail as follows and the build test is unsuccessful.

---- markdown::term::tests::test_output stdout ----
thread 'markdown::term::tests::test_output' panicked at compiler/rustc_errors/src/markdown/tests/term.rs:75:55:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: rustc_errors::markdown::term::tests::test_output
   4: <rustc_errors::markdown::term::tests::test_output::{closure#0} as core::ops::function::FnOnce<()>>::call_once
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


Running unittests src/lib.rs (/home/poky/build-st/tmp/work/core2-64-poky-linux/rust/1.76.0/rustc-1.76.0-src/build/bootstrap/debug/deps/bootstrap-214111e112557758)
uploaded "/home/poky/build-st/tmp/work/core2-64-poky-linux/rust/1.76.0/rustc-1.76.0-src/build/bootstrap/debug/deps/bootstrap-214111e112557758", waiting for result
thread 'main' panicked at src/tools/remote-test-client/src/main.rs:310:9:
client.read_exact(&mut header) failed with failed to fill whole buffer
stack backtrace:
   0: rust_begin_unwind
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:72:14
   2: remote_test_client::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: test failed, to rerun pass `--lib`
error: 1 target failed:
    `--lib`
	finished in 16.730 seconds

This failing cases should be fixed to successfully run the rustc build. Full log for more details: log_1_76_without_doc_flag.txt

https://gist.github.com/Yashinde145/036a934f0523307859f7c855b83ecfd6/raw/f7bc4558eed66a10b65bb915f292ae166d3bb52a/log_1_76_without_doc_flag.txt