Make it possible to run JSBigString tests

Summary: Add a target for JSBigString tests that can be run with a normal `buck test` invocation. Also fix an issue in the test when `getenv` returns null by defaulting to `/tmp`.

Reviewed By: ridiculousfish

Differential Revision: D14716270

fbshipit-source-id: f2eb6d3aab93c32a4b41f5786aedd04a70468d75

ReactCommon/cxxreact/tests/BUCK

@@ -55,3 +55,17 @@ fb_xplat_cxx_test(
         react_native_xplat_target("cxxreact:jsbigstring"),
     ],
 )
+
+fb_xplat_cxx_test(
+    name = "jsbigstring_test",
+    srcs = ["jsbigstring.cpp"],
+    compiler_flags = [
+        "-fexceptions",
+        "-frtti",
+    ],
+    deps = [
+        "fbsource//xplat/folly:molly",
+        "fbsource//xplat/third-party/gmock:gtest",
+        react_native_xplat_target("cxxreact:jsbigstring"),
+    ],
+)

ReactCommon/cxxreact/tests/jsbigstring.cpp

@@ -15,7 +15,10 @@ using namespace facebook::react;
 namespace {
 int tempFileFromString(std::string contents)
 {
-  std::string tmp {getenv("TMPDIR")};
+  const char *tmpDir = getenv("TMPDIR");
+  if (tmpDir == nullptr)
+    tmpDir = "/tmp";
+  std::string tmp {tmpDir};
   tmp += "/temp.XXXXXX";
 
   std::vector<char> tmpBuf {tmp.begin(), tmp.end()};