Auto merge of #7013 - servo:user-stylesheets, r=mbrubeck

Make the Ahem font available to test-css and test-wpt tests.

Add support for user stylesheets, and provide one to tests with an `@font-face` rule for it.

Fix #6195.

Many previously-failing tests now pass, and a few previously-passing now fail.

Among the latter, `font-family-013.htm` and `fonts-013.htm` are testing that the Ahem font is not used for characters it doesn’t have a glyph for. They were passing because Ahem was not available at all, and now fail because we don’t implement font fallback correctly.

The others also use Ahem, but I don’t understand yet what’s going on exactly.

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components/servo/Cargo.lock

@@ -462,6 +462,14 @@ dependencies = [
  "winapi 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "getopts"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "log 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "gfx"
 version = "0.0.1"
@@ -1521,6 +1529,7 @@ dependencies = [
  "bitflags 0.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "cssparser 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "euclid 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "getopts 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "html5ever 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "ipc-channel 0.1.0 (git+https://github.com/pcwalton/ipc-channel)",
  "js 0.1.0 (git+https://github.com/servo/rust-mozjs)",