fix test_copy_file for tmpfs #2950

Job	Run time
clang-tidy	9m 46s
check headers	3m 57s
build (asio-debugging=on picker-debugging=on)	29m 19s
build (extensions=off logging=off streaming=off super-seeding=off share-mode=off predictive-piece...	22m 36s
build dist (ubuntu-20.04)	17m 51s
build (crypto=gcrypt)	26m 22s
Fuzzers	8m 14s
build dist (ubuntu-22.04)	15m 40s
build (mmap-disk-io=off)	28m 18s
pre-commit	39s
Simulations	18m 23s
Tests (address-sanitizer=norecover undefined-sanitizer=norecover crypto=openssl)	16m 46s
Tests (toolset=clang logging=off address-sanitizer=norecover undefined-sanitizer=norecover)	12m 2s
Tests (thread-sanitizer=norecover crypto=openssl release debug-symbols=on cxxflags=-Wno-tsan)	17m 49s
Tests (crypto=gnutls)	11m 56s
Tests (deprecated-functions=off)	11m 34s
	4h 11m 12s

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