gopls/test/integration/misc: add diagnostics to flaky test

watchflakes reports that TestGOMODCACHE is flaky, presumably because
somehow the module cache index does not get written in time. This
CL adds logging information to help diagnose the failure.

Fixes: golang/go#77894
Change-Id: I03e6d9c9187c93ecb3c152d843649b1a789877a4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/753120
TryBot-Bypass: Peter Weinberger <pjw@google.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>

Author: pjweinbgo

gopls/internal/test/integration/misc/imports_test.go

@@ -255,6 +255,24 @@ var _, _ = x.X, y.Y
 
 		// Apply suggested fix via OrganizeImports.
 		env.SaveBuffer("main.go") // => OrganizeImports
+		if true {
+			// according to https://github.com/golang/go/issues/77894
+			// this test is flaky. Print some possibly helpful diagnostic
+			// information.
+			ix, err := modindex.Read(modcache)
+			if err != nil {
+				t.Logf("could not read modcache index: %v", err)
+			} else if len(ix.Entries) != 2 {
+				t.Logf("%d modcache entries", len(ix.Entries))
+				if len(ix.Entries) == 0 {
+					fis, err := os.ReadDir(modcache)
+					if err != nil {
+						t.Logf("could not read modcache dir: %v", err)
+					}
+					t.Logf("%d modcache files", len(fis))
+				}
+			}
+		}
 		env.AfterChange(NoDiagnostics(ForFile("main.go")))
 
 		// Verify that y.Y is defined within the module cache.