test: move json tests from manual/ to boost/

Manual tests are, as the name suggests, not run automatically,
which makes them more prone to regressions. JSON tests are
fast and correct, so there's no reason for them to be marked
as manual.

Message-Id: <dea75b0a0d1c238d12382a28840978884ac6ec2c.1594023481.git.sarna@scylladb.com>

configure.py

@@ -323,6 +323,7 @@ scylla_tests = set([
     'test/boost/idl_test',
     'test/boost/input_stream_test',
     'test/boost/json_cql_query_test',
+    'test/boost/json_test',
     'test/boost/keys_test',
     'test/boost/like_matcher_test',
     'test/boost/limiting_data_source_test',
@@ -391,7 +392,6 @@ scylla_tests = set([
     'test/manual/gossip',
     'test/manual/hint_test',
     'test/manual/imr_test',
-    'test/manual/json_test',
     'test/manual/message',
     'test/manual/partition_data_test',
     'test/manual/row_locker_test',
@@ -943,6 +943,7 @@ pure_boost_tests = set([
     'test/boost/enum_option_test',
     'test/boost/enum_set_test',
     'test/boost/idl_test',
+    'test/boost/json_test',
     'test/boost/keys_test',
     'test/boost/like_matcher_test',
     'test/boost/linearizing_input_stream_test',
@@ -956,7 +957,6 @@ pure_boost_tests = set([
     'test/boost/small_vector_test',
     'test/boost/top_k_test',
     'test/boost/vint_serialization_test',
-    'test/manual/json_test',
     'test/manual/streaming_histogram_test',
 ])