tools: cleanup entries from the hardcoded skip_files list in common.v (used by v test, v test-self etc); use the new // vtest build: syntax to mark the tests instead (#23918)

vlib/context/cancel_test.v

@@ -1,5 +1,6 @@
 // vtest flaky: true
 // vtest retry: 3
+// vtest build: amd64 || arm64
 import context
 
 // This example demonstrates the use of a cancelable context to prevent a

vlib/context/deadline_test.v

@@ -1,3 +1,4 @@
+// vtest build: false // TODO: it sometimes blocks, investigate why
 import context
 import time
 

vlib/context/empty_test.v

@@ -1,3 +1,4 @@
+// vtest build: amd64 || arm64
 module context
 
 fn test_background() {

vlib/context/onecontext/onecontext_test.v

@@ -1,3 +1,4 @@
+// vtest build: false // backtrace_symbols is missing
 module onecontext
 
 import context

vlib/context/value_test.v

@@ -1,3 +1,4 @@
+// vtest build: amd64 || arm64
 import context
 
 const not_found_value = &Value{