This reverts commit c98af28.

Reverts #1069

I'll redo it without removing GoTestFmt install step to fix:

#1070

Take 2 on removing gotestfmt. Since extension workflows still use it,
retain the installation stanza, but not actually use in tests.

pulumi/pulumi-azure-native#3404 is promising but
to be evaluated later I think?

Previously: #1069
#1071

Following ci-mgmt where it [was already
removed](pulumi/ci-mgmt#1069).

Fixes #2106.

## Context

We previously used
[`gotestfmt`](#1069) to improve
test output readability on CI, but it was
[removed](#1072) because it was
mis-attributing logs from one test to another and not reporting logs
when a panic was encountered — making failures harder to diagnose rather
than easier.

`gotestsum` is a more robust alternative. This PR introduces two
Makefile variables:
- `GO_TEST_EXEC ?= go test` — set to `"gotestsum --format github-actions
--"` on CI via the workflow `env:` block
- `TESTTAGS ?= all` — overridden per-language in the non-shorted matrix
path

## Why the workflow had to change too

An earlier attempt set `GO_TEST_EXEC` in the workflow env and relied on
Make's `?=` to pick it up. This worked for the **sharded path** (which
calls `make test`), but not the **non-sharded path**, which called `go
test` directly in the workflow YAML — bypassing the Makefile entirely.
awsx uses the non-sharded path, so `GO_TEST_EXEC` was never consulted.

This change moves all test invocations through `make` so `GO_TEST_EXEC`
is always honoured, and also aligns with the strategic goal of keeping
workflows "dumb" — test logic lives in the Makefile, not scattered
across workflow YAML.

## Changes

- Add `GO_TEST_EXEC ?= go test` and `TESTTAGS ?= all` to both `base` and
`parameterized-go` Makefile templates
- Use `$(GO_TEST_EXEC)` and `$(TESTTAGS)` in `test` and `test_provider`
make targets
- Set `GO_TEST_EXEC: "gotestsum --format github-actions --"` in the test
workflow's global `env:`
- Replace non-sharded `go test` workflow steps with `make
TESTTAGS=<language> GOTESTARGS=<flags> test`
- Add `gotestsum` via aqua to `mise.test.toml` so it is installed in
test workflow environments

## Test plan

- [x] `make test-go test-providers` passes — all 15 test providers
regenerate and pass actionlint
- [x] Verify gotestsum output on a real provider CI run — sample run:
pulumi/pulumi-awsx#1907
- Works like a charm! <img width="984" height="606" alt="image"
src="https://github.com/user-attachments/assets/79e08f6a-4ec0-4483-93f6-f32f6d77406a"
/>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>