Keep make test under 2 GB of memory (closes #344)

Every test that starts a database hashed the bootstrap admin password
with Argon2id at 64 MB, which under -race holds about 150 MB per hash,
across dozens of parallel tests. The test packages that hash now lower
the memory cost to 1 MB from TestMain. One test still hashes and
verifies at the shipped parameters, which are unchanged.

script/test also runs at most four packages and eight parallel tests
at once: unbounded, a many-core host linked and ran every test binary
together.

Model: opus-5-5

internal/resetpw/main_test.go

@@ -0,0 +1,14 @@
+package resetpw_test
+
+import (
+	"testing"
+
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// TestMain lowers the password hashing cost before any test runs. See
+// database.LowerPasswordHashCostForTest.
+func TestMain(m *testing.M) {
+	database.LowerPasswordHashCostForTest()
+	m.Run()
+}