Go tests run with -race and -cover under Go's own timeout (closes #88)

backend/script/test runs go test -timeout 30s -race -cover and, if
that fails, runs it again with -v and fails. The root script/test drops
its one 30-second timeout around both halves: from a cold Go build
cache, compiling the tests with -race used it all up. Each half keeps
its own limit. The race detector needs a C compiler: the Dockerfile
builder stage gains gcc and musl-dev, and script/bootstrap installs gcc,
with the C library headers on apt and apk, when gcc is missing; make
build still sets CGO_ENABLED=0. New tests: the health check's answer, a
valid report's answer, a report file's exact lines, and the flush at the
10 MiB threshold. The handlers TestImport stub is gone.

Model: opus-5-5

script/bootstrap

@@ -64,7 +64,8 @@ detect_pkgmgr() {
     fi
 }
 
-# pkg_install <nix-attr> <apt-pkg> <brew-formula> <apk-pkg>
+# pkg_install <nix-attr> <apt-pkgs> <brew-formula> <apk-pkgs>: the apt
+# and apk arguments may each list several packages, separated by spaces.
 pkg_install() {
     detect_pkgmgr
     case "$PKGMGR" in
@@ -74,10 +75,10 @@ pkg_install() {
                 $SUDO env DEBIAN_FRONTEND=noninteractive apt-get update
                 APT_UPDATED=1
             fi
-            $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y "$2"
+            $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y $2
             ;;
         brew) brew install "$3" ;;
-        apk) apk add --no-cache "$4" ;;
+        apk) apk add --no-cache $4 ;;
     esac
 }
 
@@ -253,6 +254,12 @@ main() {
 
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
+    # The race detector in make test needs cgo, which Go turns on only
+    # when it finds its C compiler, gcc on Linux. apt and apk ship the C
+    # library headers apart from gcc.
+    if missing gcc; then
+        pkg_install gcc "gcc libc6-dev" gcc "gcc musl-dev"
+    fi
 
     ensure_node
     ensure_yarn