spack audit externals: allow selecting platforms and checking extra attributes (#43782)

lib/spack/docs/packaging_guide.rst

@@ -6435,9 +6435,12 @@ the ``paths`` attribute:
          echo "Target: x86_64-pc-linux-gnu"
          echo "Thread model: posix"
          echo "InstalledDir: /usr/bin"
+     platforms: ["linux", "darwin"]
      results:
      - spec: 'llvm@3.9.1 +clang~lld~lldb'
 
+If the ``platforms`` attribute is present, tests are run only if the current host
+matches one of the listed platforms.
 Each test is performed by first creating a temporary directory structure as
 specified in the corresponding ``layout`` and by then running
 package detection and checking that the outcome matches the expected
@@ -6471,6 +6474,10 @@ package detection and checking that the outcome matches the expected
      - A spec that is expected from detection
      - Any valid spec
      - Yes
+   * - ``results:[0]:extra_attributes``
+     - Extra attributes expected on the associated Spec
+     - Nested dictionary with string as keys, and regular expressions as leaf values
+     - No
 
 """""""""""""""""""""""""""""""
 Reuse tests from other packages