Move JSON tests into a directory.

To get around the "following path contains more than 968 entries, you
should move the test to some relevant subdirectory" tidy error.

Author: nnethercote

src/test/ui/json-multiple.stderr

@@ -0,0 +1 @@
+{"artifact":"$TEST_BUILD_DIR/json/json-multiple/libjson_multiple.rlib","emit":"link"}

src/test/ui/json-options.stderr

@@ -0,0 +1 @@
+{"artifact":"$TEST_BUILD_DIR/json/json-options/libjson_options.rlib","emit":"link"}