proc_test: implement test for DWZ compressed DWARF

If dwz binary is available in the system, test delve's ability to find
deduplicated symbols in the DWARF information.

dwzcompression.go contains a small C function (void fortytwo()) which
calls glibc's fprintf with stdin as first argument. Normally, stdin
will be present as a DW_TAG_variable as part of a DW_TAG_compile_unit
named dwzcompression.cgo2.c.

After running dwz on the binary, stdin is moved to a
DW_TAG_partial_unit, which is imported from dwzcompression.cgo2.c with
a DW_TAG_imported_unit.

This test verifies that delve is able to find stdin symbol's type, as a
way to confirm it understands dwz's compressed/deduplicated DWARF
information.

_fixtures/dwzcompression.go

@@ -0,0 +1,14 @@
+package main
+
+// #include <stdio.h>
+// void fortytwo()
+// {
+//      fprintf(stdin, "42");
+// }
+import "C"
+import "runtime"
+
+func main() {
+	C.fortytwo()
+	runtime.Breakpoint()
+}

pkg/proc/proc_test.go

@@ -3697,3 +3697,20 @@ func TestIssue951(t *testing.T) {
 		}
 	})
 }
+	
+func TestDWZCompression(t *testing.T) {
+	// If dwz is not available in the system, skip this test
+	if _, err := exec.LookPath("dwz"); err != nil {
+		t.Skip("dwz not installed")
+	}
+
+	withTestProcessArgs("dwzcompression", t, ".", []string{}, protest.EnableDWZCompression, func(p proc.Process, fixture protest.Fixture) {
+		_, err := setFunctionBreakpoint(p, "C.fortytwo")
+		assertNoError(err, t, "setFunctionBreakpoint()")
+		assertNoError(proc.Continue(p), t, "first Continue()")
+		val := evalVariable(p, t, "stdin")
+		if val.RealType == nil {
+			t.Errorf("Can't find type for \"stdin\" global variable")
+		}
+	})
+}