ProcessBuilderTest: Don't depend on export shell builtin

Shells other than Bash may not have an `export` builtin command
(in particular it doesn't exist on Windows). Shells are icky anyway.
Let's make our own in one line of PHP.

I'm doing this as the first patch towards making PHPUnit pass on
Windows, because trying to run a command that doesn't exist triggers
a really unpleasant bug in amphp/process 1.x on Windows, where it
hangs forever: amphp/process#71

(I had to move `$process->getStdout()` before `$process->join()`,
otherwise that would also hang forever. No idea why.)

lib/Amp/Tests/Process/ProcessBuilderTest.php

@@ -13,7 +13,7 @@ class ProcessBuilderTest extends TestCase
 
     public function testBuildProcess(): void
     {
-        $process = ProcessBuilder::create(['export'])->build();
+        $process = $this->makeEnvDumpProcess()->build();
         $pid = wait($process->start());
         $exitCode = wait($process->join());
         self::assertEquals(0, $exitCode);
@@ -22,28 +22,33 @@ public function testBuildProcess(): void
     public function testDoesNotMergeEnvByDefaultWhenEnvVarsPassed(): void
     {
         putenv('FOO='.self::PARENT_PROCESS_ENV_VAR);
-        $process = ProcessBuilder::create(['export'])->env(['ENV'=> 'myenvvar'])->build();
+        $process = $this->makeEnvDumpProcess()->env(['ENV'=> 'myenvvar'])->build();
         $pid = wait($process->start());
-        $exitCode = wait($process->join());
-        self::assertEquals(0, $exitCode);
         /** @var string $out @phpstan-ignore-next-line */
         $out = wait(buffer($process->getStdout()));
         self::assertStringContainsString('myenvvar', $out);
         self::assertStringNotContainsString(self::PARENT_PROCESS_ENV_VAR, $out);
+        $exitCode = wait($process->join());
+        self::assertEquals(0, $exitCode);
         putenv('FOO');
     }
 
     public function testInheritsWhenInstructedToWhenEnvVarsPassed(): void
     {
         putenv('FOO='.self::PARENT_PROCESS_ENV_VAR);
-        $process = ProcessBuilder::create(['export'])->env(['ENV'=> 'myenvvar'])->mergeParentEnv()->build();
+        $process = $this->makeEnvDumpProcess()->env(['ENV'=> 'myenvvar'])->mergeParentEnv()->build();
         $pid = wait($process->start());
-        $exitCode = wait($process->join());
-        self::assertEquals(0, $exitCode);
         /** @var string $out @phpstan-ignore-next-line */
         $out = wait(buffer($process->getStdout()));
         self::assertStringContainsString('myenvvar', $out);
         self::assertStringContainsString(self::PARENT_PROCESS_ENV_VAR, $out);
+        $exitCode = wait($process->join());
+        self::assertEquals(0, $exitCode);
         putenv('FOO');
     }
+
+    private function makeEnvDumpProcess(): ProcessBuilder
+    {
+        return ProcessBuilder::create([PHP_BINARY, '-r', 'var_dump(getenv());']);
+    }
 }