Fix in TFormula the parsing of user defined functions which contain pre-defined functions (e.g. gaus) in their name. Fixes ROOT-10815.
For example when a user defines a function with the name "f1gaus" and then reuses that name could cause in same case an error parsing the expression.
Example :
```
TF1 f1("f1gaus","gaus");
TF1 f2("f2gaus","f1gaus+gaus(3)");
```
If the function name is for example  "fgaus" it was working before, but not if a character number is used before the pre-defined function name such as  "f1gaus".

Add also a test for parsing these cases in TFormulaParsingTest.

This commit fixes ROOT-10815