Fix test.json for boost test to match schema

I guess the intent was that tests of thread/debug library variants only
get run with MSVC, but currently this test isn't getting run at all in
our Windows CI (since boost got removed from the VM image [1], and we
didn't notice, more on which anon).

[1] https://github.com/actions/virtual-environments/pull/2843

test cases/frameworks/1 boost/test.json

@@ -7,10 +7,10 @@
       ],
       "b_vscrt": [
         { "val": null },
-        { "val": "md",  "compilers": { "cpp": [ "msvc" ] } },
-        { "val": "mdd", "compilers": { "cpp": [ "msvc" ] } },
-        { "val": "mt",  "compilers": { "cpp": [ "msvc" ] } },
-        { "val": "mtd", "compilers": { "cpp": [ "msvc" ] } }
+        { "val": "md",  "compilers": { "cpp": "msvc" } },
+        { "val": "mdd", "compilers": { "cpp": "msvc" } },
+        { "val": "mt",  "compilers": { "cpp": "msvc" } },
+        { "val": "mtd", "compilers": { "cpp": "msvc" } }
       ]
     },
     "exclude": [