Run c++ tests only as separate entity

6e84e4d changed the way tests were written, but doing so dropped
separation between c.segy and c++ tests.

By bringing it back we remove duplicated test runs (reducing cases run
in test entities 1-4 from 101 to 81, by 20 tests that belong to the c++
group and are not affected by mmap/lsb distinction).

Note that c++ code is built and tests are run regardless of the
"experimental" build option. That option impacts installation only.

lib/CMakeLists.txt

@@ -133,8 +133,8 @@ target_compile_definitions(c.segy
     PRIVATE
         ${mmap}
 )
-add_test(NAME c.segy          COMMAND c.segy)
-add_test(NAME c.segy.mmap     COMMAND c.segy --test-mmap)
-add_test(NAME c.segy.lsb      COMMAND c.segy --test-lsb)
-add_test(NAME c.segy.mmap.lsb COMMAND c.segy --test-mmap --test-lsb)
+add_test(NAME c.segy          COMMAND c.segy [c.segy])
+add_test(NAME c.segy.mmap     COMMAND c.segy [c.segy] --test-mmap)
+add_test(NAME c.segy.lsb      COMMAND c.segy [c.segy] --test-lsb)
+add_test(NAME c.segy.mmap.lsb COMMAND c.segy [c.segy] --test-mmap --test-lsb)
 add_test(NAME cpp.segy        COMMAND c.segy [c++])