Fix failing FileTest#test_canonicalCachesAreOff()

FileTest#test_canonicalCachesAreOff() were passing only in
vogar, in cts they were failing to create a symlink named "test_sl".

Fixed the test by making the symlink name a valid one (that can be
created without an error).

Test: FileTest in vogar and cts
Bug: 62301183
Bug: 64598164
Change-Id: I6162d1f8e8d3dc83c5dd6a5c5f4ac68ca0204685
(cherry picked from commit 07e6e4e2af1ec4c8ef4e1d70fe55664270c4f4c9)