Doc: make src/test/*/README match current reality.

Commit c3382a3c3, which moved the implementation of PG_TEST_EXTRA
from src/test/Makefile into individual test scripts, broke the
directions given in the subdirectory README files about how to run
these tests by hand.  Update.  Also mention wal_consistency_checking
in recovery/README --- that omission isn't the fault of c3382a3c3,
but it's still an omission.

src/test/kerberos/README

@@ -21,9 +21,9 @@ Running the tests
 NOTE: You must have given the --enable-tap-tests argument to configure.
 
 Run
-    make check
+    make check PG_TEST_EXTRA=kerberos
 or
-    make installcheck
+    make installcheck PG_TEST_EXTRA=kerberos
 You can use "make installcheck" if you previously did "make install".
 In that case, the code in the installation tree is tested.  With
 "make check", a temporary installation tree is built from the current

src/test/ldap/README

@@ -20,9 +20,9 @@ Running the tests
 NOTE: You must have given the --enable-tap-tests argument to configure.
 
 Run
-    make check
+    make check PG_TEST_EXTRA=ldap
 or
-    make installcheck
+    make installcheck PG_TEST_EXTRA=ldap
 You can use "make installcheck" if you previously did "make install".
 In that case, the code in the installation tree is tested.  With
 "make check", a temporary installation tree is built from the current

src/test/recovery/README

@@ -24,4 +24,9 @@ sources and then tested.
 Either way, this test initializes, starts, and stops several test Postgres
 clusters.
 
+If you want to test WAL consistency checking, add
+PG_TEST_EXTRA=wal_consistency_checking
+to the "make" command.  This is resource-intensive, so it's not done
+by default.
+
 See src/test/perl/README for more info about running these tests.

src/test/ssl/README

@@ -20,9 +20,9 @@ Also, to use "make installcheck", you must have built and installed
 contrib/sslinfo in addition to the core code.
 
 Run
-    make check
+    make check PG_TEST_EXTRA=ssl
 or
-    make installcheck
+    make installcheck PG_TEST_EXTRA=ssl
 You can use "make installcheck" if you previously did "make install".
 In that case, the code in the installation tree is tested.  With
 "make check", a temporary installation tree is built from the current