[unittests] Fix ExpandTilde test to match handling home dirs with trailing slash

The `expandTildeExpr` routine just replaces a tilde by a home dir path.
If the home dir has a trailing slash, the result of substitution will
contain double slashes. For example, `HOME=/foo/ ~/bar` gives `/foo//bar`.
That corresponds to (at least) Bash behaviour because the following
command `$HOME=/foo/ echo ~/bar` prints `/foo//bar`.

The `ExpandTilde` test constructs a path expected as the `fs::expand_tilde`
call result by calling `path::append` and the expected path has a single
slash. This patch fixes that and allows to pass the unittest on hosts where
the `HOME` is `/`.

Differential Revision: http://reviews.llvm.org/D54752

llvm-svn: 347346