Access to EEXIST through a stub getter function

This change introduces a new function `swt_EEXIST` to get the value of
`EEXIST` because it is a complex macro in wasi-libc and cannot be
imported directly into Swift.

Author: kateinoigakukun

Sources/Testing/Attachments/Test.Attachment.swift

@@ -246,7 +246,7 @@ extension Test.Attachment {
           file = try FileHandle(atPath: preferredPath, mode: "wxb")
           result = preferredPath
           break
-        } catch let error as CError where error.rawValue == EEXIST {
+        } catch let error as CError where error.rawValue == swt_EEXIST() {
           // Try again with a new suffix.
           continue
         } catch where usingPreferredName {

Sources/_TestingInternals/include/Stubs.h

@@ -154,6 +154,14 @@ static int swt_siginfo_t_si_status(const siginfo_t *siginfo) {
 #endif
 #endif
 
+/// Get the value of `EEXIST`.
+///
+/// This function is provided because `EEXIST` is a complex macro in wasi-libc
+/// and cannot be imported directly into Swift.
+static int swt_EEXIST(void) {
+  return EEXIST;
+}
+
 SWT_ASSUME_NONNULL_END
 
 #endif