README.md: mention the testsuite

6 jobs for master
Status Job ID Name Coverage
  Test
passed #209478
slow
Build and Check

02:28:34

passed #209477
fast
Build and Check with asan

00:24:35

passed #217636
fast
Build and Check with valgrind

01:42:15

passed #209475
fast
Build minimal

00:06:17

passed #209474
fast
Build with separate build dir

00:22:05

failed #209476
fast
Build and Check with valgrind

10:00:03