mtest: add support for hard errors

Hard errors also come from the GNU Automake test protocol.  They happen when
e.g., the set-up of a test case scenario fails, or when some
other unexpected or highly undesirable condition is encountered.

TAP will use them for parse errors too.  Add them to the exitcode protocol
first.

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/markdown/Unit-tests.md b/docs/markdown/Unit-tests.md
index a8e7273..9148bd5 100644
--- a/docs/markdown/Unit-tests.md
+++ b/docs/markdown/Unit-tests.md
@@ -51,10 +51,12 @@ By default Meson uses as many concurrent processes as there are cores on the tes
 $ MESON_TESTTHREADS=5 ninja test
 ```
 
-## Skipped tests
+## Skipped tests and hard errors
 
 Sometimes a test can only determine at runtime that it can not be run. The GNU standard approach in this case is to exit the program with error code 77. Meson will detect this and report these tests as skipped rather than failed. This behavior was added in version 0.37.0.
 
+In addition, sometimes a test fails set up so that it should fail even if it is marked as an expected failure. The GNU standard approach in this case is to exit the program with error code 99. Again, Meson will detect this and report these tests as `ERROR`, ignoring the setting of `should_fail`. This behavior was added in version 0.50.0.
+
 ## Testing tool
 
 The goal of the meson test tool is to provide a simple way to run tests in a variety of different ways. The tool is designed to be run in the build directory.