The web-platform-tests Project provides MathML tests for the MathML Core specification. This directory contains tests for MathML3 features implemented in Gecko or for Gecko-specific behaviors that are not described in any specification:

disabled/: Tests for MathML handling when support is disabled. This is mostly used for Tor browser's "high security" mode, see bug 1173199.
fonts: font-related tests, such as OpenType features not handled yet in MathML Core or other Gecko-specific behavior.
mathml-console-messages.html: Tests for Gecko-specific console warning and error messages triggered by MathML markup.
mathspaces_names: Tests for MathML3 namedspaces which are removed from MathML Core. See bug 1793549.
mathvariant: Tests for the mathvariant attribute, which is reduced to the case <mi mathvariant="normal"> in MathML Core. See bug 1821980.
mpadded: Tests for some mpadded features, which are not in the initial version of MathML Core.
menclose: Tests for the menclose element, which is not in the initial version of MathML Core. See issue 216.
mo-accent: Tests for the mo@accent attribute, and the corresponding accent property from the dictionary, which are removed from MathML Core. See bug 1790548 and bug 1636428.
negative-lengths: Tests for negative lengths, for which support or interpretation is unclear in current version of MathML Core. See issue 132.
operator-stretching: Tests for operator stretching, using Gecko-specific methods that are not part of the current version of MathML Core.
rtl: Tests for RTL MathML, for aspects not completely defined in MathML Core or for which we use things like scale transform for mirroring. See issue 67.
scripts: Tests for MathML scripted elements, for Gecko features that are not defined in MathML Core or in contradiction with the spec.
tables: Tests for table features that are in the initial version of MathML Core. See issue 125.
whitespace-trimming: Tests for whitespace trimming in token elements which is not described in the initial version of MathML Core. See issue 149.
zoom: Tests to check MathML rendering at different zoom levels.