Tests: metadata-regeneration: restore date at the end of the test

The metadata-regeneration test sets the date is the past to validate
that the clock offset is re-computed when regenerating a trace's
metadata.

In doing so, it leaves the time in the past which causes 'make' to panic
when it checks for changes while continuing to evaluate the 'check'
target.

  make[4]: Entering directory '/root/workspace/lttng-tools_master_rootbuild_i386/babeltrace_version/stable-2.0/build/std/conf/agents/liburcu_version/master/node/i386-rootnode/platform/deb11-i386/test_type/base/src/lttng-tools/tests/perf'
  make[4]: Warning: File '.deps/find_event.Po' has modification time 1363500490 s in the future
  ============================================================================
  Testsuite summary for lttng-tools 2.14.0-pre
  ============================================================================
  # TOTAL: 0
  # PASS:  0
  # SKIP:  0
  # XFAIL: 0
  # FAIL:  0
  # XPASS: 0
  # ERROR: 0
  ============================================================================
  make[4]: warning:  Clock skew detected.  Your build may be incomplete.
  make[4]: Leaving directory '/root/workspace/lttng-tools_master_rootbuild_i386/babeltrace_version/stable-2.0/build/std/conf/agents/liburcu_version/master/node/i386-rootnode/platform/deb11-i386/test_type/base/src/lttng-tools/tests/perf'
  make[3]: warning:  Clock skew detected.  Your build may be incomplete.
  make[3]: Leaving directory '/root/workspace/lttng-tools_master_rootbuild_i386/babeltrace_version/stable-2.0/build/std/conf/agents/liburcu_version/master/node/i386-rootnode/platform/deb11-i386/test_type/base/src/lttng-tools/tests/perf'
  make[2]: warning:  Clock skew detected.  Your build may be incomplete.
  make[2]: Leaving directory '/root/workspace/lttng-tools_master_rootbuild_i386/babeltrace_version/stable-2.0/build/std/conf/agents/liburcu_version/master/node/i386-rootnode/platform/deb11-i386/test_type/base/src/lttng-tools/tests/perf'
  make[1]: *** [Makefile:557: check-recursive] Error 1

The date is sampled at the beginning of the test and restored when it
ends. This leaves the system with a wrong time (offset by the duration
of the test itself). Still, its better than being 40 years in the past.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I4a128b57d4d9fd61eec097948b688060815f4a4c