Fix regex support in --only, --skipfile and --skiptest

The regex support was added in:
 * redis#9352
 * redis#9555
 * redis#10212

These commits break backword compatiblity with older versions.

This fix keeps the testsuite infra compatible with old versions by
default. If you want regex the string must start with `/`

tests/instances.tcl

@@ -441,7 +441,7 @@ proc run_tests {} {
             file delete $::leaked_fds_file
         }
 
-        if {[llength $::run_matching] != 0 && [search_pattern_list $test $::run_matching true] == -1} {
+        if {[llength $::run_matching] != 0 && [search_pattern_list $test $::run_matching] == -1} {
             continue
         }
         if {[file isdirectory $test]} continue

tests/support/test.tcl

@@ -126,11 +126,11 @@ proc wait_for_condition {maxtries delay e _else_ elsescript} {
     }
 }
 
-# try to match a value to a list of patterns that is either regex, or plain sub-string
-proc search_pattern_list {value pattern_list {substr false}} {
+# try to match a value to a list of patterns that are either regex (starts with "/") or plain string.
+proc search_pattern_list {value pattern_list} {
     set n 0
     foreach el $pattern_list {
-        if {[string length $el] > 0 && ((!$substr && [regexp -- $el $value]) || ($substr && [string match $el $value]))} {
+        if {[string length $el] > 0 && (([string match /* $el] && [regexp -- $el /$value]) || [string equal $el $value])} {
             return $n
         }
         incr n

tests/test_helper.tcl

@@ -588,7 +588,7 @@ proc print_help_screen {} {
         "--single <unit>    Just execute the specified unit (see next option). This option can be repeated."
         "--verbose          Increases verbosity."
         "--list-tests       List all the available test units."
-        "--only <test>      Just execute tests that match <test> regexp. This option can be repeated."
+        "--only <test>      Just execute the specified test by test name or tests that match <test> regexp. This option can be repeated."
         "--skip-till <unit> Skip all units until (and including) the specified one."
         "--skipunit <unit>  Skip one unit."
         "--clients <num>    Number of test clients (default 16)."