build/msvc: Add workaround for false positive exit code from MSBuild

See https://cmake.org/pipermail/cmake-developers/2015-October/026775.html

ci/build.ps1

@@ -91,7 +91,18 @@ cmake --build . --config $cmakeBuildType -- $cmakeGeneratorArgs ; exitIfFailed
 bin\nvim --version ; exitIfFailed
 
 # Functional tests
-cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs ; exitIfFailed
+# The $LastExitCode from MSBuild can't be trusted
+$failed = $false
+# Temporarily turn off tracing to reduce log file output
+Set-PSDebug -Off
+cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs |
+  foreach { $failed = $failed -or
+    $_ -match 'Running functional tests failed with error'; $_ }
+Set-PSDebug -Trace 1
+if ($failed) {
+  exit $LastExitCode
+}
+
 
 if ($uploadToCodecov) {
   C:\msys64\usr\bin\bash -lc "cd /c/projects/neovim; bash <(curl -s https://codecov.io/bash) -c -F functionaltest || echo 'codecov upload failed.'"