build(golang1.15): Upgrade to golang 1.15 for smaller binary (#1303)

Golang 1.15 comes to few improvements, one of them is to have smaller
binary.

This PR is to make golang 1.15 as default version in CI, I also update
Docker based image to golang:1.15* as well.

Two issues faced with golang 1.15:
- Conflict between -v in `golangci-lint` and `go test`. Update to --verbose
to avoid the same. [1]
- `nolintlint_unused.go` testdata is not matching regex. Correct by adding one
space after //

[1]: https://github.com/golang/go/issues/40763

Signed-off-by: Tam Mach <sayboras@yahoo.com>

test/enabled_linters_test.go

@@ -176,7 +176,7 @@ func TestEnabledLinters(t *testing.T) {
 		t.Run(c.name, func(t *testing.T) {
 			t.Parallel()
 
-			runArgs := []string{"-v"}
+			runArgs := []string{"--verbose"}
 			if !c.noImplicitFast {
 				runArgs = append(runArgs, "--fast")
 			}