Fix additional PowerShell warning (take two)

Since Show Session Menu always fully enumerates the iterator we needed
to move the existence check into the generator. Fortunately the 'exists'
method was already idempotent. I'd like this to be cleaner, but at least
the tests now make sense (and required a stub fix).

src/platform.ts

@@ -148,7 +148,7 @@ export class PowerShellExeFinder {
 
         // Also show any additionally configured PowerShells
         // These may be duplicates of the default installations, but given a different name.
-        for (const additionalPwsh of this.enumerateAdditionalPowerShellInstallations()) {
+        for await (const additionalPwsh of this.enumerateAdditionalPowerShellInstallations()) {
             if (await additionalPwsh.exists()) {
                 yield additionalPwsh;
             } else if (!additionalPwsh.suppressWarning) {
@@ -230,7 +230,7 @@ export class PowerShellExeFinder {
      * Iterates through the configured additional PowerShell executable locations,
      * without checking for their existence.
      */
-    private *enumerateAdditionalPowerShellInstallations(): Iterable<IPossiblePowerShellExe> {
+    private async *enumerateAdditionalPowerShellInstallations(): AsyncIterable<IPossiblePowerShellExe> {
         for (const versionName in this.additionalPowerShellExes) {
             if (Object.prototype.hasOwnProperty.call(this.additionalPowerShellExes, versionName)) {
                 let exePath: string | undefined = utils.stripQuotePair(this.additionalPowerShellExes[versionName]);
@@ -245,24 +245,44 @@ export class PowerShellExeFinder {
 
                 // Always search for what the user gave us first, but with the warning
                 // suppressed so we can display it after all possibilities are exhausted
-                yield new PossiblePowerShellExe(exePath, ...args);
+                let pwsh = new PossiblePowerShellExe(exePath, ...args);
+                if (await pwsh.exists()) {
+                    yield pwsh;
+                    continue;
+                }
 
                 // Also search for `pwsh[.exe]` and `powershell[.exe]` if missing
                 if (this.platformDetails.operatingSystem === OperatingSystem.Windows) {
                     // Handle Windows where '.exe' and 'powershell' are things
                     if (!exePath.endsWith("pwsh.exe") && !exePath.endsWith("powershell.exe")) {
                         if (exePath.endsWith("pwsh") || exePath.endsWith("powershell")) {
                             // Add extension if that was missing
-                            yield new PossiblePowerShellExe(exePath + ".exe", ...args);
+                            pwsh = new PossiblePowerShellExe(exePath + ".exe", ...args);
+                            if (await pwsh.exists()) {
+                                yield pwsh;
+                                continue;
+                            }
                         }
                         // Also add full exe names (this isn't an else just in case
                         // the folder was named "pwsh" or "powershell")
-                        yield new PossiblePowerShellExe(path.join(exePath, "pwsh.exe"), ...args);
-                        yield new PossiblePowerShellExe(path.join(exePath, "powershell.exe"), ...args);
+                        pwsh = new PossiblePowerShellExe(path.join(exePath, "pwsh.exe"), ...args);
+                        if (await pwsh.exists()) {
+                            yield pwsh;
+                            continue;
+                        }
+                        pwsh = new PossiblePowerShellExe(path.join(exePath, "powershell.exe"), ...args);
+                        if (await pwsh.exists()) {
+                            yield pwsh;
+                            continue;
+                        }
                     }
                 } else if (!exePath.endsWith("pwsh")) {
                     // Always just 'pwsh' on non-Windows
-                    yield new PossiblePowerShellExe(path.join(exePath, "pwsh"), ...args);
+                    pwsh = new PossiblePowerShellExe(path.join(exePath, "pwsh"), ...args);
+                    if (await pwsh.exists()) {
+                        yield pwsh;
+                        continue;
+                    }
                 }
 
                 // If we're still being iterated over, no permutation of the given path existed so yield an object with the warning unsuppressed