tests: more resilient about tainted absolute srcdir path

* tests/init.cfg (stty_reversible_init_): Quote '$abs_top_srcdir'
properly.
(fiemap_capable_): Quote '$abs_srcdir' properly.
(require_dirent_d_type_): Likewise.

init.cfg

@@ -266,7 +266,7 @@ stty_reversible_init_()
 {
   # Pad start with one space for the first option to match in query function.
   stty_reversible_=' '$(perl -lne '/^ *{"(.*?)",.*\bREV\b/ and print $1' \
-    $abs_top_srcdir/src/stty.c | tr '\n' ' ')
+    "$abs_top_srcdir"/src/stty.c | tr '\n' ' ')
   # Ensure that there are at least 62, i.e., so we're alerted if
   # reformatting the source empties the list.
   test 62 -le $(echo "$stty_reversible_"|wc -w)  \
@@ -398,7 +398,7 @@ fiemap_capable_()
     warn_ 'fiemap_capable_: python missing: assuming not fiemap capable'
     return 1
   fi
-  python $abs_srcdir/tests/fiemap-capable "$@"
+  python "$abs_srcdir"/tests/fiemap-capable "$@"
 }
 
 # Skip the current test if "." lacks d_type support.
@@ -413,7 +413,7 @@ require_dirent_d_type_()
   df -x xfs . > /dev/null 2>&1 \
     || skip_ requires d_type support
 
-  python $abs_srcdir/tests/d_type-check \
+  python "$abs_srcdir"/tests/d_type-check \
     || skip_ requires d_type support
 }