[libcxx] [test] Change ifstream constructor tests to handle read-only files.

Certain source control systems like to set the read-only bit on their files,
which interferes with opening "test.dat" for both input and output.
Fortunately, we can work around this without losing test coverage.
Now, the ifstream.cons tests have comments referring to the ofstream.cons tests.
There, we're creating writable files (not checked into source control),
where the ifstream constructor tests will succeed.

Fixes D26814.

llvm-svn: 289463