Color test output and shorten chapter paths

[stevecheckoway]

Currently, the output from rustdoc --test is not colored because rustdoc's stdout is not a tty. The output of a failed rustdoc run is sent to mdbook's stderr via the error!() macro. This commit checks if stderr is a tty using the standard .is_terminal() and if so, passes --color always to rustdoc.

The test output from rustdoc includes the full path that rustdoc was called with. This obfuscates the file with the error. E.g.,

---- /var/folders/9v/90bm7kb10fx3_bprxltb3t1r0000gn/T/mdbook-tnGJxp/lab0/index.md - Lab_0__Getting_Started (line 3) stdout ----
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `code`
 --> /var/folders/9v/90bm7kb10fx3_bprxltb3t1r0000gn/T/mdbook-tnGJxp/lab0/index.md:4:6
  |
3 | this code has a bug
  |      ^^^^ expected one of 8 possible tokens

This commit runs rustdoc in the temp directory and replaces any relative library paths with absolute library paths. This leads to simpler error messages. The one above becomes

---- lab0/index.md - Lab_0__Getting_Started (line 3) stdout ----
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `code`
 --> lab0/index.md:4:6
  |
3 | this code has a bug
  |      ^^^^ expected one of 8 possible tokens

error: aborting due to previous error

(with colors, of course).

[stevecheckoway]

Here's a screenshot of the output mentioned in the commit message.

[stevecheckoway]

I realized after creating this PR that I had accidentally included an unrelated change that causes mdbook to mimic how cargo test runs rustdoc by passing-C embed-bitcode=no --error-format human.

I split the commit into two different commits. The first commit makes the changes described above. The second commit just adds these command line options. I'm happy to drop the second commit, but the -C embed-bitcode=no part at least is probably useful to (very slightly) speed up tests, regardless of whether you want the colored output/shorter file paths.

[ehuss]

Thanks! I went ahead and dropped the second commit. -C embed-bitcode=no has no effect, since rustdoc is only generating executables, and those do not have bitcode. And --error-format=human is the default, so there shouldn't be a need to set that.