[lldb][test] Enable TestExpressionInSyscall.py on Windows

Relates to #22139

Just had to use the right win32 call instead of getpid. The original
problem was something with expressions in general which was
fixed at some point.

Also make the test one method, there's no need to split it up.

Author: DavidSpickett

lldb/test/API/commands/expression/expr-in-syscall/TestExpressionInSyscall.py

@@ -7,16 +7,10 @@
 
 
 class ExprSyscallTestCase(TestBase):
-    @expectedFailureAll(
-        oslist=["windows"],
-        bugnumber="llvm.org/pr21765, getpid() does not exist on Windows",
-    )
     @expectedFailureNetBSD
     def test_setpgid(self):
         self.build()
-        self.expr_syscall()
 
-    def expr_syscall(self):
         # Create a target by the debugger.
         target = self.createTestTarget()
 
@@ -65,7 +59,12 @@ def expr_syscall(self):
 
         # try evaluating a couple of expressions in this state
         self.expect_expr("release_flag = 1", result_value="1")
-        self.expect_expr("(int)getpid()", result_value=str(process.GetProcessID()))
+        func = (
+            "GetCurrentProcessId"
+            if lldbplatformutil.getPlatform() == "windows"
+            else "getpid"
+        )
+        self.expect_expr(f"(int){func}()", result_value=str(process.GetProcessID()))
 
         # and run the process to completion
         process.Continue()