Add T8_ENABLE_MINIMAL_TESTS

[lukasdreyer]

Feature request

Is your feature request related to a problem? Please describe.
I'm always frustrated when I implement new features in the core and the testsuite runs too long.

Describe the solution or feature you'd like
I'd like to have the option to run a reduced testsuite covering only extreme cmeshes for example, where all tests combined run in under a minute using
CMAKE_CUSTOM_PARALLEL_TEST_COMMAND=mpirun -n 4, CTEST_PARALLEL_LEVEL=8, and the tests are executed using ninja test -j32.

Describe alternatives you've considered
Using GTEST_FILTER to only run specific tests, but often the failing tests are the long ones, and they would also fail, if we reduced the tests to a smaller subset of the generated tests.

[lukasdreyer]

These are the tests currently preventing fast execution of all tests:

11/68 Test #33: t8_gtest_forest_commit_parallel ....................... Passed 16.04 sec
13/68 Test #31: t8_gtest_ghost_and_owner_parallel ..................... Passed 16.23 sec
19/68 Test #29: t8_gtest_ghost_exchange_parallel ...................... Passed 18.44 sec
31/68 Test #32: t8_gtest_balance_parallel ............................. Passed 10.62 sec
66/68 Test #50: t8_gtest_nca_serial ................................... Passed 42.39 sec
67/68 Test #35: t8_gtest_element_is_leaf_serial ....................... Passed 55.40 sec
68/68 Test #39: t8_gtest_iterate_replace_serial ....................... Passed 56.26 sec