[LibOS/regression] Add test for host root FS

Graphene supports a special "root FS" parameter in the manifest:
"fs.root". This commit adds a test that uses this parameter.

LibOS/shim/test/regression/host_root_fs.c

@@ -0,0 +1,31 @@
+#include <dirent.h>
+#include <stdio.h>
+
+static int showdir(char* path) {
+    struct dirent* de;
+
+    DIR* dir = opendir(path);
+    if (!dir) {
+        printf("Could not open directory `%s`\n", path);
+        return 1;
+    }
+
+    printf("Contents of directory `%s`:\n", path);
+    while ((de = readdir(dir)))
+        printf("  %s\n", de->d_name);
+    printf("\n");
+
+    closedir(dir);
+    return 0;
+}
+
+int main(int argc, char** argv) {
+    if (showdir("/"))
+        return 1;
+
+    if (showdir("/var/"))
+        return 1;
+
+    puts("Test was successful");
+    return 0;
+}

LibOS/shim/test/regression/host_root_fs.manifest.template

@@ -0,0 +1,16 @@
+loader.preload = file:../../src/libsysdb.so
+loader.env.LD_LIBRARY_PATH = /lib
+loader.debug_type = none
+loader.syscall_symbol = syscalldb
+
+fs.root.type = chroot
+fs.root.path = /
+fs.root.uri = file:/
+
+fs.mount.graphene_lib.type = chroot
+fs.mount.graphene_lib.path = /lib
+fs.mount.graphene_lib.uri = file:../../../../Runtime
+
+sgx.trusted_files.ld = file:../../../../Runtime/ld-linux-x86-64.so.2
+sgx.trusted_files.libc = file:../../../../Runtime/libc.so.6
+sgx.trusted_files.libdl = file:../../../../Runtime/libdl.so.2

LibOS/shim/test/regression/test_libos.py

@@ -251,6 +251,10 @@ class TC_30_Syscall(RegressionTestCase):
 
         self.assertIn('Success!', stdout)
 
+    def test_022_host_root_fs(self):
+        stdout, _ = self.run_binary(['host_root_fs'])
+        self.assertIn('Test was successful', stdout)
+
     def test_030_fopen(self):
         if os.path.exists("tmp/filecreatedbygraphene"):
             os.remove("tmp/filecreatedbygraphene")