.github/workflows: fix natlab test discovery with fully-excluded files

The Discover tests step runs under set -euo pipefail, so when every
Test function in a file is filtered out by the exclude regex (as with
vnetperf_test.go, whose TestVnetPerf* tests need special invocation),
the final grep -vE produces no output and exits 1, failing the whole
step. Tolerate empty results from both greps so such files simply
contribute no matrix entries. Same for a hypothetical future test
file containing only helpers and no Test functions.

Updates tailscale/corp#44805

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: If20c9a1de37e97b1553bd7d5559e2b876a45c19b

.github/workflows/natlab-test.yml

@@ -108,9 +108,9 @@ jobs:
             pkg="./${pkg_dir}/"
             for f in "${pkg_dir}"/*_test.go; do
               [ -e "$f" ] || continue
-              grep -hE '^func Test[A-Z][A-Za-z0-9_]*\(t \*testing\.T\)' "$f" \
+              { grep -hE '^func Test[A-Z][A-Za-z0-9_]*\(t \*testing\.T\)' "$f" || true; } \
                 | sed -E 's/^func (Test[A-Za-z0-9_]+).*/\1/' \
-                | grep -vE "$exclude" \
+                | { grep -vE "$exclude" || true; } \
                 | while read -r t; do
                     jq -nc --arg pkg "$pkg" --arg test "$t" \
                       '{pkg: $pkg, test: $test}' >> "$tmp"