[clang-tidy][test] Add more bugprone-string-constructor tests (#70456)

Test:

* string literal concatenation is accepted
* escaped embedded null char is accepted
* taking over the implicit null char is rejected

Author: maflcko

clang-tools-extra/test/clang-tidy/checkers/bugprone/string-constructor.cpp

@@ -49,6 +49,8 @@ void Test() {
   // CHECK-MESSAGES: [[@LINE-1]]:15: warning: negative value used as length parameter
   std::string q2("test", 200);
   // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger than string literal size
+  std::string t1("test", 5);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger than string literal size
   std::string q3(kText, 200);
   // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger than string literal size
   std::string q4(kText2, 200);
@@ -97,6 +99,8 @@ void Valid() {
   std::string s1("test", 4);
   std::string s2("test", 3);
   std::string s3("test");
+  std::string s4("test\000", 5);
+  std::string s6("te" "st", 4);
 
   std::string_view emptyv();
   std::string_view sv1("test", 4);