Add extra tests to threadtest4.c.  Fix a benign data race accessing the
text encoding using ENC(db).

FossilOrigin-Name: d7bb7ea4ab97ad26f4c84c9b8dc2827010093803
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 80f9597..d90089b 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -1059,6 +1059,7 @@
   int errCode;                  /* Most recent error code (SQLITE_*) */
   int errMask;                  /* & result codes with this before returning */
   u16 dbOptFlags;               /* Flags to enable/disable optimizations */
+  u8 enc;                       /* Text encoding */
   u8 autoCommit;                /* The auto-commit flag. */
   u8 temp_store;                /* 1: file 2: memory 0: default */
   u8 mallocFailed;              /* True if we have seen a malloc failure */
@@ -1160,7 +1161,8 @@
 /*
 ** A macro to discover the encoding of a database.
 */
-#define ENC(db) ((db)->aDb[0].pSchema->enc)
+#define SCHEMA_ENC(db) ((db)->aDb[0].pSchema->enc)
+#define ENC(db)        ((db)->enc)
 
 /*
 ** Possible values for the sqlite3.flags.