tests: prevent the test from being joined

The local header include no longer works when the test is part of
the megatest.

Author: zerbina

tests/lang/s05_pragmas/s01_interop/t07_imported_struct_at_compile_time.nim

@@ -3,6 +3,7 @@ discard """
     Imported C struct types can be used in compile-time evaluation contexts.
   '''
   targets: "c"
+  joinable: false
 """
 
 type Extern {.importc: "struct NoTypedef", header: "t01_c_interop.nim.h".} = object