Fix Windows platform detection, Watchman detection in tests

Summary:
While checking some `metro-file-map` behaviour on Windows I noticed the Watchman detection we use in integration tests was all wrong :)
 - `os.platform()` returns `win32` on Windows, not `windows`.
 - `where` works in a command prompt but not PowerShell - there it's usually an alias for `Where-Object`. `where.exe` works on both.
 - `where.exe` prints "INFO: Could not find files for the given pattern(s)" to stderr on no match. Use `/Q` for silent running - the exit code is all we need.

Reviewed By: motiz88

Differential Revision: D42266748

fbshipit-source-id: 4494cd39dd94da7dc17442264a3828a35aaf9c30

packages/metro-file-map/src/crawlers/__tests__/integration-test.js

@@ -21,7 +21,9 @@ import type {CrawlerOptions, FileData} from '../../flow-types';
 // and will tend to exercise our Watchman tests whenever possible.
 const isWatchmanOnPath = () => {
   try {
-    execSync(os.platform() === 'windows' ? 'where watchman' : 'which watchman');
+    execSync(
+      os.platform() === 'win32' ? 'where.exe /Q watchman' : 'which watchman',
+    );
     return true;
   } catch {
     return false;

packages/metro-file-map/src/watchers/__tests__/helpers.js

@@ -31,7 +31,9 @@ const {mkdtemp, writeFile} = fsPromises;
 // and will tend to exercise our Watchman tests whenever possible.
 const isWatchmanOnPath = () => {
   try {
-    execSync(os.platform() === 'windows' ? 'where watchman' : 'which watchman');
+    execSync(
+      os.platform() === 'win32' ? 'where.exe /Q watchman' : 'which watchman',
+    );
     return true;
   } catch {
     return false;