internal/lsp/regtest: minor cleanup for magic regtest envvar

For some reason, we were not reusing the constant runTestAsGoplsEnvVar
in the regtest Main function. Fix this, add a bit more commentary, and
check the envvar before doing anything else. Also fix the threading of
hooks to the created gopls server.

Tested manually via -enable_gopls_subprocess_tests.

Change-Id: Ieb5329aa5850e845f4d9e3868703bfa16387bce3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/420716
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

internal/lsp/regtest/regtest.go

@@ -94,16 +94,19 @@ func DefaultModes() Mode {
 
 // Main sets up and tears down the shared regtest state.
 func Main(m *testing.M, hook func(*source.Options)) {
+	// If this magic environment variable is set, run gopls instead of the test
+	// suite. See the documentation for runTestAsGoplsEnvvar for more details.
+	if os.Getenv(runTestAsGoplsEnvvar) == "true" {
+		tool.Main(context.Background(), cmd.New("gopls", "", nil, hook), os.Args[1:])
+		os.Exit(0)
+	}
+
 	testenv.ExitIfSmallMachine()
 
 	// Disable GOPACKAGESDRIVER, as it can cause spurious test failures.
 	os.Setenv("GOPACKAGESDRIVER", "off")
 
 	flag.Parse()
-	if os.Getenv("_GOPLS_TEST_BINARY_RUN_AS_GOPLS") == "true" {
-		tool.Main(context.Background(), cmd.New("gopls", "", nil, nil), os.Args[1:])
-		os.Exit(0)
-	}
 
 	runner = &Runner{
 		DefaultModes:             DefaultModes(),