[functional tests] Move test output from /tmp to .

tmpfs has trouble with O_DIRECT.

README.md

@@ -124,7 +124,7 @@ Then,
 
 Other command are help and list.
 
-The test framework places temporary files under /tmp/test-output/.  By default
+The test framework places temporary files under ./test-output/.  By default
 the tests tidy up after themselves, just leaving a log file for each test.  You
 can turn this off by using the --disable-unlink flag if you want all the
 artifacts left.

functional-tests/temp-file.scm

@@ -20,7 +20,7 @@
           (only (srfi s1 lists) span))
 
   ;; FIXME: global var!  Not thread safe.
-  (define working-dir "/tmp")
+  (define working-dir ".")
 
   (define (working-directory) working-dir)