Hadrian: Update README about the flavour/testsuite contract

There have been a number of tickets about non-tested flavours not
passing the testsuite.. this is expected and now noted in the
documentation. You use other flavours to run the testsuite at your own
risk.

Fixes #21418

hadrian/README.md

@@ -193,6 +193,11 @@ To run GHC testsuite, use `build test`. See
 flags, as well as about the equivalents of the features that the Make build
 system offers.
 
+NOTE: The only build flavours which are expected to pass the testsuite are those
+tested in CI. If you use an untested flavour such as "Quick" then you run the
+risk that not all tests will pass. In particular you can rely on the `validate`
+and `perf` flavours being tested but no others.
+
 `build selftest` runs tests of the build system. The current test coverage
 is close to zero (see [#197][test-issue]).
 

hadrian/doc/testsuite.md

@@ -17,6 +17,11 @@ built GHC before, this will also build a stage 2 GHC in
 the default flavour along with many libraries and programs
 needed by the tests.
 
+NOTE: The only build flavours which are expected to pass the testsuite are those
+tested in CI. If you use an untested flavour such as "Quick" then you run the
+risk that not all tests will pass. In particular you can rely on the `validate`
+and `perf` flavours being tested but no others.
+
 ## Running only a subset of the testsuite
 
 ### Specific tests