tracing: Build event generation tests only as modules

The kprobes and synth event generation test modules add events and lock
(get a reference) those event file reference in module init function,
and unlock and delete it in module exit function. This is because those
are designed for playing as modules.

If we make those modules as built-in, those events are left locked in the
kernel, and never be removed. This causes kprobe event self-test failure
as below.

[   97.349708] ------------[ cut here ]------------
[   97.353453] WARNING: CPU: 3 PID: 1 at kernel/trace/trace_kprobe.c:2133 kprobe_trace_self_tests_init+0x3f1/0x480
[   97.357106] Modules linked in:
[   97.358488] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 6.9.0-g699646734ab5-dirty #14
[   97.361556] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
[   97.363880] RIP: 0010:kprobe_trace_self_tests_init+0x3f1/0x480
[   97.365538] Code: a8 24 08 82 e9 ae fd ff ff 90 0f 0b 90 48 c7 c7 e5 aa 0b 82 e9 ee fc ff ff 90 0f 0b 90...

kernel/trace/Kconfig

@@ -1136,7 +1136,7 @@ config PREEMPTIRQ_DELAY_TEST
 
 config SYNTH_EVENT_GEN_TEST
 	tristate "Test module for in-kernel synthetic event generation"
-	depends on SYNTH_EVENTS
+	depends on SYNTH_EVENTS && m
 	help
           This option creates a test module to check the base
           functionality of in-kernel synthetic event definition and
@@ -1149,7 +1149,7 @@ config SYNTH_EVENT_GEN_TEST
 
 config KPROBE_EVENT_GEN_TEST
 	tristate "Test module for in-kernel kprobe event generation"
-	depends on KPROBE_EVENTS
+	depends on KPROBE_EVENTS && m
 	help
           This option creates a test module to check the base
           functionality of in-kernel kprobe event definition.