use /tmp/test-mounts

This is so it's not inside the bind-mounted volume, otherwise tests will
fail on Docker for Mac.  (extended attributes are not supported on
Docker for Mac's bind mounts, and tests will try to set xattrs on the
lower paths.)

t/Makefile.am

@@ -74,7 +74,7 @@ EXTRA_DIST = README.md chainlint.sed \
 LOG_DRIVER = env AM_TAP_AWK='$(AWK)' HARNESS_ACTIVE=true \
 	$(SHELL) ../tap-driver.sh
 
-TEST_MOUNTS_DIR = test-mounts
+TEST_MOUNTS_DIR = /tmp/test-mounts
 TEST_OUTPUT_DIR = test-output
 PROVE_FILE = .prove
 

t/test-lib.sh

@@ -835,7 +835,7 @@ then
 fi
 
 # Test mount points
-TRASH_DIRECTORY="test-mounts/$(basename "$0" .t)"
+TRASH_DIRECTORY="/tmp/test-mounts/$(basename "$0" .t)"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good