Support negative --test-threads/--jobs parameter

[sunshowers]

Port over rust-lang/cargo#10844.

This is a great first issue and should take no more than a few hours to complete :) Here's the recommended series of steps:

1. Update the TestThreads parser and deserializer to handle negative integers:

   nextest/nextest-runner/src/config.rs

   Lines 526 to 534 in 58999b1

   	/// Type for the test-threads config key.
   	#[derive(Clone, Copy, Debug, Eq, PartialEq)]
   	pub enum TestThreads {
   	/// Run tests with a specified number of threads.
   	Count(usize),
   	/// Run tests with a number of threads equal to the logical CPU count.
   	NumCpus,
   	}

   
   Use the logic at https://github.com/rust-lang/cargo/pull/10844/files#diff-f0c6e0ba24dc1403233aec133c8788a36c21b097514ea69994ff35253a07802a.
2. Add unit tests for this.

[OLUWAMUYIWA]

Interested in this. I will do the porting

[sunshowers]

@OLUWAMUYIWA awesome, thanks! Please comment here if you run into issues.

[OLUWAMUYIWA]

Will do. Thanks @sunshowers