add basic tests to check compatibility with REL_11 in travis

Author: alubennikova

.travis.yml

@@ -27,3 +27,5 @@ env:
     - PG_VERSION=12 PG_BRANCH=REL_12_STABLE TEST_CASE=all MODE=paranoia
     - PG_VERSION=12 PG_BRANCH=REL_12_STABLE TEST_CASE=test_ptrack_multiple_segments TEST_REPEATS=5 MODE=paranoia
     - PG_VERSION=12 PG_BRANCH=REL_12_STABLE TEST_CASE=test_ptrack_eat_my_data TEST_REPEATS=4
+    - PG_VERSION=11 PG_BRANCH=REL_11_STABLE TEST_CASE=tap
+    - PG_VERSION=11 PG_BRANCH=REL_11_STABLE TEST_CASE=tap MODE=legacy

codecov.yml

@@ -1,6 +1,6 @@
 codecov:
   notify:
-    after_n_builds: 5 # keep in sync with .travis.yml number of builds
+    after_n_builds: 8 # keep in sync with .travis.yml number of builds
 
 # datapagemap.c/.h are copied from Postgres, so let's remove it
 # from report.  Otherwise, we would have to remove some currently