fuse integration: point MOUNT_POINT at TEST_MOUNT_POINT

Targets that work against MOUNT_POINT (pjdfstest, nfstest, fio configs)
ignored TEST_MOUNT_POINT and ran against the local-disk default while the
go tests used the external mount.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

test/fuse_integration/posix_Makefile

@@ -13,6 +13,9 @@ COVERAGE_FILE := posix_coverage.out
 REPORT_DIR := reports
 EXTERNAL_TOOLS_DIR := external_tools
 MOUNT_POINT ?= /tmp/seaweedfs_mount
+ifneq ($(TEST_MOUNT_POINT),)
+MOUNT_POINT := $(TEST_MOUNT_POINT)
+endif
 
 # Go test command prefix for external mount points
 GO_TEST_PREFIX :=