src/lib/log/tests/Makefile.am: check-local -> check_SCRIPTS,TESTS

Closes #2172 (closed).

Required:

• Pass all shell tests to autoconf's check_SCRIPTS and TESTS variables through the temporary SHTESTS variable.
• Remove them from check-local so that they don't run twice at this point.
• Replace TESTSCXXFLAGS, TESTSCPPFLAGS, TESTSLDADD, TESTSLDFLAGS with AM_* variables. The variables starting with TESTS are autoconf-specific variables that resulted in the shell scripts that were newly added to TESTS to be built like C++ files, which made the build fail.

Optional:

• Split noinst_PROGRAMS in PROGRAM_TESTS and TEST_HELPERS.
• Remove extra entry in .gitignore.
• Solve death test warning.
  • Not done, because the suggested fix of adding GTEST_FLAG_SET(death_test_style, "threadsafe"); fails for me with GTEST_FLAG_SET was not declared in this scope.

Change can be tested with make check -j 1234. It should be easy to get interleaving between shell tests and C++ tests in the old version. And you should get sequential running in this version. Order is deterministic, but arbitrarily so - shell tests run first, C++ tests afterwards.

distcheck test run: https://jenkins.aws.isc.org/view/Kea-manual/job/kea-manual/job/distcheck/51/console