Update docs. The expr_string.t failure has not been seen since
the workaround was added AFAICT.  The async filter bug
is still breaking the tests regularly. [skip ci].
@@ -28,6 +28,10 @@ The Travis scripts use the following environment variables:
* SKIP_TESTING - if set, the Perl test framework is not run for the
build.
+* TEST_UBSAN - set for builds using UBSan ("Undefined Behaviour Sanitizer")
+
+* TEST_MALLOC - set for builds using enhanced malloc debugging.
+
Caching
-------
@@ -49,7 +53,6 @@ TODO list
- not sure how exactly
* Known test failures
- "apt-get install" timeout/fails - workaround by moving apt install to before_script phase?
- - t/apache/expr_string.t - unclear root cause, possible workaround in r1872705
- t/apache/rwrite.t, t/apache/pass_brigade.t
see msg <CAKQ1sVMbDcUouu+_bfp8T5HO5+=ORWhLx361+b4qyLux5xyhwA@mail.gmail.com> etc
