test: generate a minimal RPM in pure Go instead of a committed fixture

Removes the committed .rpm binary (gitignored, so it broke clean checkouts)
and the rpmbuild dependency. testsupport.MinimalRPM builds a valid-enough
RPM header in-memory for cavaliergopher/rpm to parse, used by both the rpm
provider test and the full-stack server repodata test.

internal/server/server_test.go

@@ -301,10 +301,7 @@ func TestServerLocalPyPI(t *testing.T) {
 
 func TestServerLocalRPMRepodata(t *testing.T) {
 	requireStack(t)
-	rpm, err := os.ReadFile("../provider/rpm/testdata/e2e-testpkg-1.0-1.noarch.rpm")
-	if err != nil {
-		t.Skipf("rpm fixture unavailable: %v", err)
-	}
+	rpm := testsupport.MinimalRPM("e2e-testpkg", "1.0", "1", "noarch")
 	if resp, b := req(t, "POST", "/api/v2/remotes", `{"name":"srv-rpm","package_type":"rpm","repo_type":"local"}`); resp.StatusCode != 201 {
 		t.Fatalf("create rpm local: %d %s", resp.StatusCode, b)
 	}