Test creates a file named `string` in the root source directory

[pcc]

Dart commit 5bf9bdb

tools/sdks/dart-sdk/bin/dart tools/test.dart -n vm-linux-release-arm64
# change a source file
ninja -C out/ReleaseARM64

Expected behavior: build succeeds

Actual behavior: lots of errors, e.g.

../../string:1:15: error: source file is not valid UTF-8

This appears to be caused by one of the tests creating a file named string in the root source directory which the compiler thinks is the C++ <string> header.