web: uniquify temp file paths in tests to avoid PID-collision races

tempHtml/tempPng wrote to /tmp/web_test_<label>.{html,png} with no
per-process suffix. Concurrent invocations of the same gtest case
(CI parallel jobs / retries / sharding) raced on the same path:
one process's std::ofstream truncated another's in-flight write,
producing a half-written file that was missing varying substrings
depending on flush timing.

Append getpid() to the filename so each process owns a private path.
Verified by reproducing the race with 32 parallel runs of
SaveReportTest.ContainsInlinedJS (100% failure before, 0% after).

Signed-off-by: Matt Liberty <mliberty@precisioninno.com>

Author: maliberty

src/web/test/cpp/TestSaveImage.cpp

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: BSD-3-Clause
 // Copyright (c) 2026, The OpenROAD Authors
 
+#include <unistd.h>
+
 #include <cstddef>
 #include <cstring>
 #include <filesystem>
@@ -79,8 +81,9 @@ class SaveImageTest : public tst::Nangate45Fixture
   // Save to a temp file and register for cleanup.
   std::string tempPng(const std::string& label)
   {
-    std::string path = std::filesystem::temp_directory_path()
-                       / ("web_test_" + label + ".png");
+    std::string path
+        = std::filesystem::temp_directory_path()
+          / ("web_test_" + label + "_" + std::to_string(::getpid()) + ".png");
     output_files_.push_back(path);
     return path;
   }

src/web/test/cpp/TestSaveReport.cpp

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: BSD-3-Clause
 // Copyright (c) 2026, The OpenROAD Authors
 
+#include <unistd.h>
+
 #include <cstddef>
 #include <filesystem>
 #include <fstream>
@@ -59,8 +61,9 @@ class SaveReportTest : public tst::Nangate45Fixture
 
   std::string tempHtml(const std::string& label)
   {
-    std::string path = std::filesystem::temp_directory_path()
-                       / ("web_test_" + label + ".html");
+    std::string path
+        = std::filesystem::temp_directory_path()
+          / ("web_test_" + label + "_" + std::to_string(::getpid()) + ".html");
     output_files_.push_back(path);
     return path;
   }