test: resolve a few issues with tests causing inconsistent results (#5992)

* test: correct the usage of runInstallFixture helper

* test: use serial when calling getPort

Author: mrstork

packages/build/tests/install/snapshots/tests.js.md

@@ -673,8 +673,7 @@ Generated by [AVA](https://avajs.dev).
       repositoryRoot: packages/build/tests/install/fixtures/yarn␊
       testOpts:␊
         pluginsListUrl: test␊
-        silentLingeringProcesses: true␊
-      useBinary: true␊
+        silentLingeringProcesses: 'true'␊
     ␊
     > Current directory␊
       packages/build/tests/install/fixtures/yarn␊
@@ -730,13 +729,11 @@ Generated by [AVA](https://avajs.dev).
     ␊
     > Flags␊
       debug: true␊
-      flags:␊
-        mode: buildbot␊
+      mode: buildbot␊
       repositoryRoot: packages/build/tests/install/fixtures/yarn_ci␊
       testOpts:␊
         pluginsListUrl: test␊
-        silentLingeringProcesses: true␊
-      useBinary: true␊
+        silentLingeringProcesses: 'true'␊
     ␊
     > Current directory␊
       packages/build/tests/install/fixtures/yarn_ci␊

packages/build/tests/install/snapshots/tests.js.snap

@@ -112,14 +112,18 @@ test('Install local plugin dependencies: with npm', async (t) => {
 })
 
 test('Install local plugin dependencies: with yarn locally', async (t) => {
-  await runInstallFixture(t, 'yarn', [`${FIXTURES_DIR}/yarn/plugin/node_modules/`], { useBinary: true })
+  await runInstallFixture(t, 'yarn', [`${FIXTURES_DIR}/yarn/plugin/node_modules/`], {}, true, true)
 })
 
 test('Install local plugin dependencies: with yarn in CI', async (t) => {
-  await runInstallFixture(t, 'yarn_ci', [`${FIXTURES_DIR}/yarn_ci/plugin/node_modules/`], {
-    useBinary: true,
-    flags: { mode: 'buildbot' },
-  })
+  await runInstallFixture(
+    t,
+    'yarn_ci',
+    [`${FIXTURES_DIR}/yarn_ci/plugin/node_modules/`],
+    { mode: 'buildbot' },
+    true,
+    true,
+  )
 })
 
 test('Install local plugin dependencies: propagate errors', async (t) => {

packages/build/tests/install/tests.js

@@ -368,7 +368,7 @@ test('Plugin errors that occur during the loading phase are piped to system logs
   t.snapshot(normalizeOutput(output))
 })
 
-test('Plugins have a pre-populated Blobs context', async (t) => {
+test.serial('Plugins have a pre-populated Blobs context', async (t) => {
   const serverPort = await getPort()
   const deployId = 'deploy123'
   const siteId = 'site321'