tests: s/GITEA_UNIT_TESTS_VERBOSE/GITEA_UNIT_TESTS_LOG_SQL/ (#18142)

The GITEA_UNIT_TESTS_VERBOSE variable is an undocumented variable
introduced in 2017 (see 1028ef2def)
whose sole purpose has been to log SQL statements when running unit
tests.

It is renamed for clarity and a warning is displayed for backward
compatibility for people and scripts that know about it.

The documentation is updated to reflect this change.

models/unittest/testdb.go

@@ -152,7 +152,7 @@ func CreateTestEngine(opts FixturesOptions) error {
 	if err = db.SyncAllTables(); err != nil {
 		return err
 	}
-	switch os.Getenv("GITEA_UNIT_TESTS_VERBOSE") {
+	switch os.Getenv("GITEA_UNIT_TESTS_LOG_SQL") {
 	case "true", "1":
 		x.ShowSQL(true)
 	}