Run ztest for longer on "Coverage" builders

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes openzfs#6675

TEST

@@ -83,6 +83,20 @@ Ubuntu*)
     ;;
 esac
 
+###
+#
+# Run ztest longer on the "coverage" builders to gain more code coverage
+# data out of ztest, libzpool, etc.
+#
+case "$BB_NAME" in
+*coverage*)
+    TEST_ZTEST_TIMEOUT=3600
+    ;;
+*)
+    TEST_ZTEST_TIMEOUT=900
+    ;;
+esac
+
 ###
 #
 # Disable the following test suites on 32-bit systems.