Fix CI: json_view tests without exceptions, on clang 3.6, and single header

- ci_test_noexceptions: the helpers that compare the exceptions of
  json_document::parse() and json::parse() catch them outside a
  CHECK_THROWS, so with JSON_NOEXCEPTION the first parse error aborted
  the test. Compile those comparisons only with exceptions, as
  unit-class_parser.cpp does.
- ci_test_gcc: -Werror=unused-result for CHECK_THROWS_AS(json_document::
  parse(...)); assign the result to a dummy document.
- ci_test_compilers_clang (3.6): `const json_view invalid;` needs a
  user-provided default constructor there (CWG 253); value-initialize it.
- ci_test_single_header: json_view.hpp now exists as a single header and
  contains the internal view headers, so unit-json_view_builder.cpp
  includes it instead of the detail headers in that mode, and the test
  is built again with the single header.
- Regenerate single_include/nlohmann/json_view.hpp for the builder change
  merged from json-view/08-view-builder.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_builder.cpp

@@ -9,8 +9,12 @@
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
-#include <nlohmann/detail/view/builder.hpp>
-#include <nlohmann/detail/view/string_ref.hpp>
+#if JSON_TEST_USING_MULTIPLE_HEADERS
+    #include <nlohmann/detail/view/builder.hpp>
+    #include <nlohmann/detail/view/string_ref.hpp>
+#else
+    #include <nlohmann/json_view.hpp> // the single header contains the internal headers
+#endif
 using nlohmann::json;
 
 #include <cstdint>