Only clear exit test ID env var upon successful lookup (#1226)

This changes the logic in `ExitTest.findInEnvironmentForEntryPoint()` to
only clear the environment variable containing the ID of the exit test
to run if an exit test is successfully located.

### Motivation:

This ensures that if a tool integrates with the testing library and has
both its own built-in copy of the library but also may call into the ABI
entry point, both usage scenarios can successfully look up the exit
test. Without this fix, if an earlier attempt to look up an exit test
fails, the environment variable would be cleared which prevents a
subsequent lookup attempt from succeeding.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Sources/Testing/ExitTests/ExitTest.swift

@@ -233,6 +233,10 @@ extension ExitTest {
 #endif
   }
 
+  /// The name of the environment variable used to identify the exit test to
+  /// call in a spawned exit test process.
+  private static let _idEnvironmentVariableName = "SWT_EXIT_TEST_ID"
+
   /// Call the exit test in the current process.
   ///
   /// This function invokes the closure originally passed to
@@ -713,6 +717,17 @@ extension ExitTest {
 #endif
   }
 
+  /// The ID of the exit test to run, if any, specified in the environment.
+  static var environmentIDForEntryPoint: ID? {
+    guard var idString = Environment.variable(named: Self._idEnvironmentVariableName) else {
+      return nil
+    }
+
+    return try? idString.withUTF8 { idBuffer in
+      try JSON.decode(ExitTest.ID.self, from: UnsafeRawBufferPointer(idBuffer))
+    }
+  }
+
   /// Find the exit test function specified in the environment of the current
   /// process, if any.
   ///
@@ -723,21 +738,15 @@ extension ExitTest {
   /// `__swiftPMEntryPoint()` function. The effect of using it under other
   /// configurations is undefined.
   static func findInEnvironmentForEntryPoint() -> Self? {
-    // Find the ID of the exit test to run, if any, in the environment block.
-    var id: ExitTest.ID?
-    if var idString = Environment.variable(named: "SWT_EXIT_TEST_ID") {
-      // Clear the environment variable. It's an implementation detail and exit
-      // test code shouldn't be dependent on it. Use ExitTest.current if needed!
-      Environment.setVariable(nil, named: "SWT_EXIT_TEST_ID")
-
-      id = try? idString.withUTF8 { idBuffer in
-        try JSON.decode(ExitTest.ID.self, from: UnsafeRawBufferPointer(idBuffer))
-      }
-    }
-    guard let id, var result = find(identifiedBy: id) else {
+    guard let id = environmentIDForEntryPoint, var result = find(identifiedBy: id) else {
       return nil
     }
 
+    // Since an exit test was found, clear the environment variable. It's an
+    // implementation detail and exit test code shouldn't be dependent on it.
+    // Use ExitTest.current if needed!
+    Environment.setVariable(nil, named: Self._idEnvironmentVariableName)
+
     // If an exit test was found, inject back channel handling into its body.
     // External tools authors should set up their own back channel mechanisms
     // and ensure they're installed before calling ExitTest.callAsFunction().
@@ -867,7 +876,7 @@ extension ExitTest {
       // Insert a specific variable that tells the child process which exit test
       // to run.
       try JSON.withEncoding(of: exitTest.id) { json in
-        childEnvironment["SWT_EXIT_TEST_ID"] = String(decoding: json, as: UTF8.self)
+        childEnvironment[Self._idEnvironmentVariableName] = String(decoding: json, as: UTF8.self)
       }
 
       typealias ResultUpdater = @Sendable (inout ExitTest.Result) -> Void