AssertEqualFiles asserts two files are equal.

ChDir changes the working directory for the duration of the test.

DownloadablesPath path to testdata/downloadables.

MustReadFile reads a file or fails.

MustWriteFile write a file or fails.

ProjectPath exchanges a path relative to the project root for an absolute path.

ProjectRoot returns the absolute path of the project root.

ServeFile starts an http server.

TmpDir returns the path to a newly created tmp dir and a function for deleting that dir.

FooChecksum is the checksum of downloadablesPath("foo.tar.gz").