Color test output and shorten chapter paths

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 obscures the path of 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

error: aborting due to previous error
```

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).

src/book/mod.rs

@@ -15,8 +15,9 @@ pub use self::init::BookBuilder;
 pub use self::summary::{parse_summary, Link, SectionNumber, Summary, SummaryItem};
 
 use log::{debug, error, info, log_enabled, trace, warn};
-use std::io::Write;
-use std::path::PathBuf;
+use std::ffi::OsString;
+use std::io::{IsTerminal, Write};
+use std::path::{Path, PathBuf};
 use std::process::Command;
 use std::string::ToString;
 use tempfile::Builder as TempFileBuilder;
@@ -259,10 +260,18 @@ impl MDBook {
     /// Run `rustdoc` tests on a specific chapter of the book, linking against the provided libraries.
     /// If `chapter` is `None`, all tests will be run.
     pub fn test_chapter(&mut self, library_paths: Vec<&str>, chapter: Option<&str>) -> Result<()> {
-        let library_args: Vec<&str> = (0..library_paths.len())
-            .map(|_| "-L")
-            .zip(library_paths.into_iter())
-            .flat_map(|x| vec![x.0, x.1])
+        let cwd = std::env::current_dir()?;
+        let library_args: Vec<OsString> = library_paths
+            .into_iter()
+            .flat_map(|path| {
+                let path = Path::new(path);
+                let path = if path.is_relative() {
+                    cwd.join(path).into_os_string()
+                } else {
+                    path.to_path_buf().into_os_string()
+                };
+                [OsString::from("-L"), path]
+            })
             .collect();
 
         let temp_dir = TempFileBuilder::new().prefix("mdbook-").tempdir()?;
@@ -289,6 +298,7 @@ impl MDBook {
             .collect();
         let (book, _) = self.preprocess_book(&TestRenderer)?;
 
+        let color_output = std::io::stderr().is_terminal();
         let mut failed = false;
         for item in book.iter() {
             if let BookItem::Chapter(ref ch) = *item {
@@ -314,7 +324,10 @@ impl MDBook {
                 tmpf.write_all(ch.content.as_bytes())?;
 
                 let mut cmd = Command::new("rustdoc");
-                cmd.arg(&path).arg("--test").args(&library_args);
+                cmd.current_dir(temp_dir.path())
+                    .arg(&chapter_path)
+                    .arg("--test")
+                    .args(&library_args);
 
                 if let Some(edition) = self.config.rust.edition {
                     match edition {
@@ -330,6 +343,10 @@ impl MDBook {
                     }
                 }
 
+                if color_output {
+                    cmd.args(&["--color", "always"]);
+                }
+
                 debug!("running {:?}", cmd);
                 let output = cmd.output()?;