Add btests for fd limit-capping behavior at Zeek startup.

testing/btest/Baseline/core.fd-limits-2/output

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+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+100
+100

testing/btest/Baseline/core.fd-limits-3/output

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+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+warning in <...>/init-bare.zeek, line 1: reduced open file descriptor limits (soft/hard 1500000/2000000 -> 1048576/1048576)
+1048576
+1048576

testing/btest/Baseline/core.fd-limits-4/output

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+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+1500000
+2000000

testing/btest/Baseline/core.fd-limits-5/output

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+1500000
+2000000

testing/btest/Baseline/core.fd-limits/output

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+100
+150

testing/btest/core/fd-limits.sh

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+# Check the file descriptor limit behavior Zeek applies at startup.
+# Zeek's built-in default limit is 1024 * 1024 = 1048576 open fds.
+
+# @TEST-REQUIRES: ulimit -n -S >limit.soft
+# @TEST-REQUIRES: ulimit -n -H >limit.hard
+# @TEST-EXEC: unset ZEEK_NOFILE_MAX; LIM_SOFT=$(cat limit.soft) LIM_HARD=$(cat limit.hard) bash %INPUT
+# @TEST-EXEC: if [ ! -f skip ]; then TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff output; fi
+
+# @TEST-START-FILE limits.zeek
+event zeek_init()
+      {
+      # Report resulting limits to stderr:
+      system("ulimit -n -S");
+      system("ulimit -n -H");
+      }
+# @TEST-END-FILE
+
+# @TEST-START-FILE limits.sh
+setcaps() {
+    local soft=$1
+    local hard=$2
+
+    ulimit -S -n $soft || {
+        echo "Skipping $TEST_NAME, could not set soft fd limit $soft."
+        touch skip
+        exit 0
+    }
+    ulimit -H -n $hard || {
+        echo "Skipping $TEST_NAME, could not set hard fd limit $hard."
+        touch skip
+        exit 0
+    }
+}
+# @TEST-END-FILE
+
+# Apply low limits and no ZEEK_NOFILE_MAX variable.
+# The limits should stay unchanged with no warning message.
+
+. ./limits.sh
+setcaps 100 150
+zeek -b limits.zeek 2>output
+
+# @TEST-START-NEXT
+# Establish a low limit via ZEEK_NOFILE_MAX.
+# Zeek should apply these values, without a warning message.
+
+# Verify that one can actually set these values, then revert to defaults.
+setcaps 100 100
+setcaps $LIM_SOFT $LIM_HARD
+
+export ZEEK_NOFILE_MAX=100
+zeek -b limits.zeek 2>output
+
+# @TEST-START-NEXT
+# With high limits and no ZEEK_NOFILE_MAX Zeek should cap and produce a
+# warning message.
+
+. ./limits.sh
+setcaps 1500000 2000000
+zeek -b limits.zeek 2>output
+
+# @TEST-START-NEXT
+# With high limits and the capping mechanism disabled, the values should remain intact.
+
+. ./limits.sh
+setcaps 1500000 2000000
+export ZEEK_NOFILE_MAX=
+zeek -b limits.zeek 2>output
+
+# @TEST-START-NEXT
+# With high limits and the capping mechanism disabled, the values should remain intact.
+
+. ./limits.sh
+setcaps 1500000 2000000
+export ZEEK_NOFILE_MAX=0
+zeek -b limits.zeek 2>output
+