Partly, for correctness. But mostly because not typecasting properly
causes the gettid() macro provided by newer glibc to be typed as pid_t
(aka int), while ours is typed as long. Causing annoying warnings:

[158/846] Compiling C object 'tests/59830eb@@drm_import_export@exe/drm_import_export.c.o'.
In file included from ../../mnt/vol/lib/drmtest.h:39,
                 from ../../mnt/vol/lib/igt.h:27,
                 from ../../mnt/vol/tests/drm_import_export.c:27:
../../mnt/vol/tests/drm_import_export.c: In function ‘test_thread’:
../../mnt/vol/tests/drm_import_export.c:123:12: warning: format ‘%ld’ expects argument of type ‘long int’, but argument 4 has type ‘__pid_t’ {aka ‘int’} [-Wformat=]
  123 |  igt_debug("start %ld\n", gettid());
      |            ^~~~~~~~~~~~~  ~~~~~~~~
      |                           |
      |                           __pid_t {aka int}
../../mnt/vol/lib/igt_core.h:875:64: note: in definition of macro ‘igt_debug’
  875 | #define igt_debug(f...) igt_log(IGT_LOG_DOMAIN, IGT_LOG_DEBUG, f)
      |                                                                ^
../../mnt/vol/tests/drm_import_export.c:123:21: note: format string is defined here
  123 |  igt_debug("start %ld\n", gettid());
      |                   ~~^
      |                     |
      |                     long int
      |                   %d

So, typecast gettid() as pid_t and update all of our callers accordingly

Reviewed-by: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>