Revert "cmocka: Drop as meson subproject"

This reverts commit 99eca0899b.

The environment provided by our Jenkins builder doesn't have cmocka.
It's not exactly known why it worked as a subproject. However, if it
allows us to enforce tests in the CI, we should revert now and drop
the subproject again once the environment is updated.

Change-Id: Iafdab92eb8ab96d02fb2c29524d785bdc7a54034
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67345
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

meson.build

@@ -500,7 +500,11 @@ endif
 # `.allowed()` gets introduced in 0.59.0
 if get_option('tests').auto() or get_option('tests').enabled()
   # unit-test framework
-  cmocka = dependency('cmocka', required : get_option('tests'))
+  cmocka_dep = dependency(
+    'cmocka',
+    fallback: ['cmocka', 'cmocka_dep'],
+    required : get_option('tests')
+  )
 
   flashrom_test_dep = declare_dependency(
     include_directories : include_dir,
@@ -519,7 +523,7 @@ if get_option('tests').auto() or get_option('tests').enabled()
     ],
   )
 
-  if cmocka.found()
+  if cmocka_dep.found()
     subdir('tests')
   endif
 endif