[libc++][NFC] Make AssertionInfoMatcher::CheckMessageMatches Stricter (#77721)

Rather than allow for a message to be considered a match for the actual
assertion if it is anywhere in the assertion text, make sure that the
expected and the actual assertion are identical.

Addresses #77701

Author: hawkinsw

libcxx/test/libcxx/containers/views/mdspan/layout_left/assert.stride.pass.cpp

@@ -35,7 +35,7 @@ int main(int, char**) {
   {
     std::layout_left::mapping<std::dextents<int, 3>> m{std::dextents<int, 3>{100, 100, 100}};
 
-    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "invalid rank index");
+    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "layout_left::mapping::stride(): invalid rank index");
   }
   return 0;
 }

libcxx/test/libcxx/containers/views/mdspan/layout_right/assert.stride.pass.cpp

@@ -35,7 +35,7 @@ int main(int, char**) {
   {
     std::layout_right::mapping<std::dextents<int, 3>> m{std::dextents<int, 3>{100, 100, 100}};
 
-    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "invalid rank index");
+    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "layout_right::mapping::stride(): invalid rank index");
   }
   return 0;
 }

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.stride.pass.cpp

@@ -30,7 +30,7 @@ int main(int, char**) {
     std::layout_stride::mapping<std::dextents<int, 3>> m(
         std::dextents<int, 3>(100, 100, 100), std::array<int, 3>{1, 100, 10000});
 
-    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "invalid rank index");
+    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "layout_stride::mapping::stride(): invalid rank index");
   }
   return 0;
 }

libcxx/test/support/check_assertion.h

@@ -89,8 +89,7 @@ struct AssertionInfoMatcher {
     std::size_t found_at = got_msg.find(msg_);
     if (found_at == std::string_view::npos)
       return false;
-    // Allow any match
-    return true;
+    return found_at == 0 && got_msg.size() == msg_.size();
   }
 private:
   bool is_empty_;