[CMAKE] Rename secp256k1 test targets

Summary:
Running the secp256k1 tests is obvious, using a check-secp256k1-*
target. But building the target without running the tests is much more
complicated, since names like `tests` doesn't tell much.
This diff renames the targets to prefix with `secp256k1-*` just like we
do with `bitcoin-*`.

Test Plan:
  ninja secp256k1-tests
  ninja secp256k1-exhaustive_tests
  ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6313

src/secp256k1/CMakeLists.txt

@@ -370,15 +370,15 @@ if(SECP256K1_BUILD_TEST)
 		link_secp256k1_internal(${NAME})
 	endfunction()
 
-	create_secp256k1_test(tests src/tests.c)
-	create_secp256k1_test(exhaustive_tests src/tests_exhaustive.c)
+	create_secp256k1_test(secp256k1-tests src/tests.c)
+	create_secp256k1_test(secp256k1-exhaustive_tests src/tests_exhaustive.c)
 
 	# This should not be enabled at the same time as coverage is.
 	# The VERIFY failure branch is not expected to be reached, so it would make
 	# coverage appear lower if set.
 	if(NOT SECP256K1_ENABLE_COVERAGE)
-		target_compile_definitions(tests PRIVATE VERIFY)
-		target_compile_definitions(exhaustive_tests PRIVATE VERIFY)
+		target_compile_definitions(secp256k1-tests PRIVATE VERIFY)
+		target_compile_definitions(secp256k1-exhaustive_tests PRIVATE VERIFY)
 	endif()
 
 	if(SECP256K1_ENABLE_JNI)