test: Use script file for sed basename test.

As of this commit, Gash does not process the quotes around the script
properly, and ends up calling sed with no arguments.

test/100-sed-autoconf-basename.sh

@@ -1,15 +1 @@
-basename='/^.*\/\([^/][^/]*\)\/*$/{
-	    s//\1/
-	    q
-	  }
-  /^X\/\(\/\/\)$/{
-	    s//\1/
-	    q
-  }
-  /^X\/\(\/\).*/{
-	    s//\1/
-	    q
-  }
-  s/.*/./; q'
-
-echo 'X/foo/bar' | \sed "$basename"
+echo 'X/foo/bar' | \sed -f test/data/basename.sed

test/data/basename.sed

@@ -0,0 +1,13 @@
+/^.*\/\([^/][^/]*\)\/*$/{
+	    s//\1/
+	    q
+	  }
+  /^X\/\(\/\/\)$/{
+	    s//\1/
+	    q
+  }
+  /^X\/\(\/\).*/{
+	    s//\1/
+	    q
+  }
+  s/.*/./; q