shtest+configure: remove uses of non-portable/non-standard commands

Replace deprecated test(1) parentheses and -a logical operator with two
tests command.

Replace deprecated  tail -1  with  tail -n1.

Replace non-standard egrep(1) command with  grep -E  ; this also
prevents obsolescence warnings on GNU systems.

tests/shtest

@@ -244,7 +244,7 @@ if which seq > /dev/null 2>&1; then
         if [ -n "$VALGRIND" ]; then
             grep '^==[0-9][0-9]*== ERROR SUMMARY: 0 errors' $d/err > /dev/null
         else
-            tail -1 $d/err | egrep -i 'assert|abort|core' && false
+            tail -n1 -- "$d/err" | grep -Ei 'assert|abort|core' && false
         fi
 
         dd "if=tests/torture/input0.json" bs=$i count=1 2>/dev/null |
@@ -252,7 +252,7 @@ if which seq > /dev/null 2>&1; then
         if [ -n "$VALGRIND" ]; then
             grep '^==[0-9][0-9]*== ERROR SUMMARY: 0 errors' $d/err > /dev/null
         else
-            tail -1 $d/err | egrep -i 'assert|abort|core' && false
+            tail -n1 -- "$d/err" | grep -Ei 'assert|abort|core' && false
         fi
 
         diff $d/out0 $d/out1