Bug 1768301 - Don't run stylo tests in coverage builds for now. r=dshin

This should un-bust ccov builds for now. See comment for the follow-up
work.

Differential Revision: https://phabricator.services.mozilla.com/D146088

toolkit/library/rust/moz.build

@@ -22,13 +22,20 @@ RUST_TESTS = [
     "l10nregistry",
     "selectors",
     "servo_arc",
-    "stylo_tests",
     "qcms",
     # this does nothing, but is needed so that `gkrust_features` are recognized
     # also see: https://github.com/rust-lang/cargo/issues/9856
     "gkrust",
 ]
 
+# Code coverage builds link a bunch of Gecko bindings code from the style
+# crate, which is not used by our tests but would cause link errors.
+#
+# Bug 1768874 tracks improving the setup to potentially remove these
+# altogether.
+if not CONFIG["MOZ_CODE_COVERAGE"]:
+    RUST_TESTS += ["stylo_tests"]
+
 if CONFIG["CPU_ARCH"] != "x86":
     # malloc_size_of_derive is a build dependency, so if we are doing
     # cross-compile for x86, this may not run correctly.