gio/tests: Add testcase for bug 787731

Add testcase for function g_file_query_filesystem_info()
reporting outdated info for "filesystem::readonly" attribute
when said attribute was different in a previous mounted
partition in the same device (as GIO maintains a mounts cache
per 'st_dev' stat() member).

To trigger a mount operation, testcase uses program 'bindfs'
instead of 'mount --bind' as bindfs does not require root
privileges. And 'fusermount -u' command is used to unmount said
bindfs mount.

As a reference in Fedora, 'bindfs' is installed from 'bindfs'
package and 'fusermount' from 'fuse' package (this one is installed
by default as being part of 'System Tools' group).

The test creates a directory with a file in it, then mounts it
readonly over another directory (the mountpoint), it then checks
that g_file_query_filesystem_info() for the file in it indeed reports
"filesystem::readonly" as TRUE. Then unmounts and mounts again this
time rw (not readonly), it then checks again if g_file_query_filesystem_info()
is reporting "filesystem::readonly" as TRUE, if that's the case, it
confirms the bug by opening said file in write mode.

Testcase is only added for Unix builds.

https://bugzilla.gnome.org/show_bug.cgi?id=787731