testing: do not allow fn main in tests and make sure there is at least one test fn

compiler/main.v

@@ -429,8 +429,15 @@ string _STR_TMP(const char *fmt, ...) {
 				exit(1)
 			}
 		}
-		// Generate `main` which calls every single test function
 		else if v.pref.is_test {
+			if v.table.main_exists() {
+				cerror('test files cannot have function `main`')
+			}	
+			// make sure there's at least on test function
+			if !v.table.has_at_least_one_test_fn() {
+				cerror('test files need to have at least one test function')
+			}	
+			// Generate `main` which calls every single test function
 			cgen.genln('int main() { init_consts();')
 			for _, f in v.table.fns {
 				if f.name.starts_with('test_') {

compiler/table.v

@@ -672,6 +672,15 @@ fn (t &Table) main_exists() bool {
 	return false
 }
 
+fn (t &Table) has_at_least_one_test_fn() bool {
+	for _, f in t.fns {
+		if f.name.starts_with('test_') {
+			return true
+		}	
+	}
+	return false
+}
+
 fn (t &Table) find_const(name string) ?Var {
 	//println('find const l=$t.consts.len')
 	for c in t.consts {

compiler/tests/module_test.v

@@ -8,7 +8,7 @@ import (
 	crypto.sha512 as s5
 )
 
-pub fn main() {
+fn test_import() {
 	assert os.SUCCESS == os.SUCCESS &&
 		t.MonthDays[0] == t.MonthDays[0] &&
 		s2.Size == s2.Size &&