[6.2] Ensure NonisolatedNonsendingByDefault doesn't break exit tests (#1421)

- **Explanation**: Adjust the (non-ABI-affecting) signature of an
underlying function used by the expansion of the
`#expect(processExitsWith:)` macro so that when Approachable Concurrency
is enabled, we pick the correct overload of said function.
- **Scope**: Exit tests with Approachable Concurrency enabled.
- **Issues**: #1375
- **Original PRs**:
#1383, #1407
- **Risk**: Low
- **Testing**: Existing CI jobs and manual verification. It is not
possible at this time to write a unit test for this fix due to
swiftlang/swift-package-manager#9293.
- **Reviewers**: @grynspan @briancroom @jerryjrchen

Author: stmontgomery

Sources/Testing/ExitTests/ExitTest.swift

@@ -381,6 +381,7 @@ extension ExitTest {
 #if compiler(>=6.2)
   @safe
 #endif
+  @_disfavoredOverload
   public static func __store<T>(
     _ id: (UInt64, UInt64, UInt64, UInt64),
     _ body: T,