[tests] avoid g++ in JCDBs, even if not used

tests/jcdb/compile_commands.json

 [
     { "directory": ".",
-      "command": "g++ -DDUPLICATE_FLAGS_THAT_WILL_BE_OVERWRITTEN",
+      "command": "gcc -DDUPLICATE_FLAGS_THAT_WILL_BE_OVERWRITTEN",
       "file": "jcdb.c"
     },
     { "directory": ".",
-      "command": "g++ -DDUPLICATE_FLAGS_THAT_WILL_BE_OVERWRITTEN",
+      "command": "gcc -DDUPLICATE_FLAGS_THAT_WILL_BE_OVERWRITTEN",
       "file": "jcdb.c"
     },
     {
@@ -12,7 +12,7 @@
       "file": "jcdb.c"
     },
     { "directory": ".",
-      "command": "/usr/bin/clang++ -D'MSG=\"a \\\" \\\"b\"'  -D'SINGLE_DOUBLE(a)=\"a \\\"with spaces and	tab \"' -DSOMEDEF=\"With spaces, quotes and \\-es.\" -D\"DOUBLE_SINGLE(a)=a \\\"macro with spaces and non-escaped \\\\'\\\"\"   -DEMPTY=''  -DEMPTY2= -DTEST=42	 -D'MACRO_FOR_INCR(s)=s+1' -DTOUNDEF -UTOUNDEF",
+      "command": "/usr/bin/clang -D'MSG=\"a \\\" \\\"b\"'  -D'SINGLE_DOUBLE(a)=\"a \\\"with spaces and	tab \"' -DSOMEDEF=\"With spaces, quotes and \\-es.\" -D\"DOUBLE_SINGLE(a)=a \\\"macro with spaces and non-escaped \\\\'\\\"\"   -DEMPTY=''  -DEMPTY2= -DTEST=42	 -D'MACRO_FOR_INCR(s)=s+1' -DTOUNDEF -UTOUNDEF",
       "file": "jcdb.c"
     }
 ]

tests/jcdb/subdir.json

 [
     { "directory": ".",
-      "command": "g++ -Isubdir2",
+      "command": "gcc -Isubdir2",
       "file": "subdir.c"
     }
 ]