libtest: Fix bugs in assert_file_has_content()

First, we need to preserve whitespace in the regexps.  Second,
in the error path, all of our variables were wrong.

Commit 376a2cc3f5 is an excellent
poster child for how many bugs one can introduce in a single commit
for a ~5 line function.

Closes: #777
Approved by: jlebon

tests/common/libtest.sh

@@ -143,10 +143,10 @@ assert_not_has_dir () {
 assert_file_has_content () {
     fpath=$1
     shift
-    for re in $@; do
+    for re in "$@"; do
         if ! grep -q -e "$re" "$fpath"; then
-            sed -e 's/^/# /' < "$1" >&2
-            echo 1>&2 "File '$1' doesn't match regexp '$2'"
+            sed -e 's/^/# /' < "$fpath" >&2
+            echo 1>&2 "File '$fpath' doesn't match regexp '$re'"
             exit 1
         fi
     done