gopls/internal/test/marker: add a test for initialization cycle errors

CL 565838 changed the way initialization cycle errors were reporting,
and would have broken gopls' parsing of continuation errors, yet no
x/tools tests failed.

Add a test for this behavior. Due to golang/go#65877, duplicate
diagnostics had to be suppressed for this test.

Updates golang/go#65877

Change-Id: I48244ac469ab78d2e40bf92ec061671cef72c6d9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/566075
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: findleyr

gopls/internal/test/marker/testdata/diagnostics/initcycle.txt

@@ -0,0 +1,17 @@
+This test verifies that gopls spreads initialization cycle errors across
+multiple declarations.
+
+We set -ignore_extra_diags due to golang/go#65877: gopls produces redundant
+diagnostics for initialization cycles.
+
+-- flags --
+-ignore_extra_diags
+
+-- p.go --
+package p
+
+var X = Y //@diag("X", re"initialization cycle")
+
+var Y = Z //@diag("Y", re"initialization cycle")
+
+var Z = X //@diag("Z", re"initialization cycle")