CI: Ensure tmp directory exists on Windows before running tests

Author: snej

.github/workflows/cmake.yml

@@ -62,4 +62,5 @@ jobs:
       run: |
         set -e
         cmake --build . --config Debug --target FleeceTests
+        mkdir -p /c/tmp
         Debug/FleeceTests.exe

Tests/EncoderTests.cc

@@ -688,6 +688,7 @@ class EncoderTests {
 
         {
             FILE *out = fopen(kTempDir"fleecetemp.fleece", "wb");
+            REQUIRE(out != nullptr);
             Encoder fenc(out);
             fenc.writeValue(root);
             fenc.end();