tests/lowres_clock_test.cc: fix compilation error for older GCC
tests/lowres_clock_test.cc: fix compilation error for older GCC

A compilation error reported on the mailing list [1] indicates that older
versions of gcc may be incorrectly compiling a `static constexpr` declaration
inside of a function:

```
FAILED: build/debug/tests/lowres_clock_test.o
g++ -MD -MT build/debug/tests/lowres_clock_test.o -MF build/debug/tests/lowres_clock_test.o.d -fsanitize=address -fsanitize=leak -fsanitize=undefined -fno-sanitize=vptr -DASAN_ENABLED -O0 -DDEBUG -DDEBUG_SHARED_PTR -DDEFAULT_ALLOCATOR -DSEASTAR_THREAD_STACK_GUARDS -I/opt/src/seastar/build/debug/gen -I/opt/src/seastar/build/debug/c-ares -std=gnu++1y -g  -Wall -Werror -Wno-error=deprecated-declarations -fvisibility=hidden  -pthread -I/opt/src/seastar -U_FORTIFY_SOURCE  -I/opt/src/seastar/fmt -DBOOST_TEST_DYN_LINK -Wno-overloaded-virtual -Wno-maybe-uninitialized -DFMT_HEADER_ONLY -DHAVE_HWLOC -DHAVE_NUMA -DHAVE_LZ4_COMPRESS_DEFAULT -c -o build/debug/tests/lowres_clock_test.o tests/lowres_clock_test.cc
/tmp/ccv4zuIO.s: Assembler messages:
/tmp/ccv4zuIO.s:19986: Error: symbol `_ZL14sleep_duration' is already defined
```

I can reproduce the failure and its fix on Ubuntu 16.04 [2] and Fedora 23 [3],
though it's not clear why moving the static constant into the body of
`do_with()` resolves the error.

Interestingly, the failure is only present in debug mode.

[1] https://groups.google.com/d/msgid/seastar-dev/22372182-97d8-484f-8032-00e1e2941920%40googlegroups.com?utm_medium=email&utm_source=footer

[2] g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

[3] g++ (GCC) 5.3.1 20160406 (Red Hat 5.3.1-6)

Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <843c75585078ef27ad6974c089ede6a11bd81ac5.1499816656.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

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